Well, I'm neither part of the ObjectScript nor the Objects developer team, hence I can't answer the "why" part of your question but fact is, timing mesuremenst show a significat higher speeds for the literal versions:
 

ClassMethod DynObject()
{
	while $zh#1 {} set t1=$zh for i=1:1:1E6 { if ##class(%DynamicArray).%New()  } set t1=$zh-t1
	while $zh#1 {} set t2=$zh for i=1:1:1E6 { if ##class(%DynamicObject).%New() } set t2=$zh-t2
	while $zh#1 {} set t3=$zh for i=1:1:1E6 { if [] } set t3=$zh-t3
	while $zh#1 {} set t4=$zh for i=1:1:1E6 { if {} } set t4=$zh-t4
	
	write "Times for :    Class  Literal     Diff",!
	write "DynArray  :", $j(t1,9,3), $j(t3,9,3), $j(t1/t3-1*100,9,2),"%",!
	write "DynObject :", $j(t2,9,3), $j(t4,9,3), $j(t2/t4-1*100,9,2),"%",!
}

The output will depend on
- IRIS/Cache version in use and
- on the underlying hardware
My values are

USER>d ##class(DC.Times).DynObject()
Times for :    Class  Literal     Diff
DynArray  :    0.665    0.401    65.90%
DynObject :    0.649    0.401    61.87%

Maybe someone else or the WRC has an explanation...

JavaScript uses underscores as separators in (big)number literals.

<script>alert(1_234);</script> displays 1234
<script>alert(12_345_678_901_234_567_890_123n);</script>  displays 12345678901234567890123

Maybe there is a quirk/problem in VS terminal...

Try it this way...

Class DC.OldStuff Extends %Persistent [ StorageStrategy = NewStorage1 ]
{

Property Rec As %Integer [ Identity ];
Property Name As %String;
Property City As %String;
Property Phone As %String;
Storage NewStorage1
{
<SQLMap name="Map1">
<Data name="City">
<RetrievalCode>s {*}=$zel(^myGlo("R",{L2}),2)</RetrievalCode>
</Data>
<Data name="Name">
<RetrievalCode>s {*}=$zel(^myGlo("R",{L2}),1)</RetrievalCode>
</Data>
<Data name="Phone">
<RetrievalCode>s {*}=$zel(^myGlo("R",{L2}),3)</RetrievalCode>
</Data>
<Global>^myGlo</Global>
<Subscript name="1">
<Expression>"R"</Expression>
</Subscript>
<Subscript name="2">
<Expression>{Rec}</Expression>
</Subscript>
<Type>data</Type>
</SQLMap>
<StreamLocation>^DC.OldS</StreamLocation>
<Type>%CacheSQLStorage</Type>
}

}

A short test shows, it works

USER>k ^myGlo
USER>s ^myGlo("R",1)=$zlp("John,Boston,11-22-33")
USER>s ^myGlo("R",5)=$zlp("Laura,New York,333-444-555")
USER>s ^myGlo("R",7)=$zlp("Paul,Chicago,556-666-777")
USER>d $system.SQL.Shell()
SQL Command Line Shell
----------------------------------------------------

The command prefix is currently set to: <<nothing>>.
Enter q to quit, ? for help.
USER>>select * from DC.OldStuff
3.      select * from DC.OldStuff

ID      City    Name    Phone   Rec
1       Boston  John    11-22-33        1
5       New York        Laura   333-444-555     5
7       Chicago Paul    556-666-777     7
3 Rows(s) Affected
statement prepare time(s)/globals/lines/disk: 0.0003s/5/159/0ms
          execute time(s)/globals/lines/disk: 0.0003s/13/1136/0ms
                          cached query class: %sqlcq.USER.cls43
---------------------------------------------------------------------------
USER>>quit

or as objects

USER>s obj=##class(DC.OldStuff).%OpenId(7)

USER>w obj.Name,!,obj.City,!,obj.Phone
Paul
Chicago
556-666-777
USER>

First step: create your own method, for example

Class DC.Unix [ Abstract ]
{
/// Convert Posix time into Timestamp
/// 
/// posix: posix time
///    df: date format
///    tf: time format
///    dp: decimal places
ClassMethod PosixToTimeStamp(posix, df = 3, tf = 1, dp = 0)
{
	set posix=posix-1152921504606846976/1E6
	quit $zdt(posix\86400+47117_","_(posix#86400),df,tf,dp)
}
}

Next step, if you need more speed, instead of the parameters <df> and <tf> use constants and remove <dp>. The very last step: shorten the method name from PosixToTimeStamp() to P2TS()

This way I got the (nearly) the same times as with (%Library.PosixTime).LogicalToTimeStamp, but without the need for string manipulation 

ICINDY:USER>k

ICINDY:USER>set posix = 1154669852181849976

ICINDY:USER>while $zh#1 {} s t=$zh f i=1:1:1E6 { s x=##class(%Library.PosixTime).LogicalToTimeStamp(posix) } w $zh-t
.902538
ICINDY:USER>while $zh#1 {} s t=$zh f i=1:1:1E6 { s x=##class(%Library.PosixTime).LogicalToTimeStamp(posix) } w $zh-t
.90609
ICINDY:USER>

ICINDY:USER>while $zh#1 {} s t=$zh f i=1:1:1E6 { s x=##class(DC.Unix).PosixToTimeStamp(posix) } w $zh-t
.934834
ICINDY:USER>while $zh#1 {} s t=$zh f i=1:1:1E6 { s x=##class(DC.Unix).PosixToTimeStamp(posix) } w $zh-t
.944418
ICINDY:USER>

ICINDY:USER>while $zh#1 {} s t=$zh f i=1:1:1E6 { s x=##class(DC.Unix).P2TS(posix) } w $zh-t
.913609
ICINDY:USER>while $zh#1 {} s t=$zh f i=1:1:1E6 { s x=##class(DC.Unix).P2TS(posix) } w $zh-t
.905303
ICINDY:USER>

ICINDY:USER>w $zv
IRIS for UNIX (Ubuntu Server LTS for x86-64) 2021.2 (Build 649U) Thu Jan 20 2022 08:49:51 EST
ICINDY:USER>w x
2025-05-27 12:06:15
ICINDY:USER>

I think, Enrico was more aiming for the point that “concatenation is the gateway for SQL injection.”

An easy and simple (assuming standard storage strategy) way to get those informations (the slot numbers, where a property is stored) could be achieved with a simple classmethod

Class DC.PropInfo [ Abstract ]
{

/// Info about properties of a class:
/// - list of all properties stored in a list
/// - slot number for a given property
/// 
/// 1) add this class to your class definition
///    class some.class extends (%Persistent, DC.StorageInfo)  or
///    class some.class extends (%SerialClass, DC.StorageInfo)
///           
/// 2) then use it as follows
///    write ##class(some.class).PropInfo()      --> list of property names
///    write ##class(some.class).PropInfo("Age") --> slot number for the Age property
/// 
/// write ##class(
ClassMethod PropInfo(name = "") As %String [ CodeMode = objectgenerator ]
{
	set sto=%compiledclass.Storages, prp=0, list=""
	
	if sto.Count()=1 {
		set dat=sto.GetAt(1).Data
		for i=1:1:dat.Count() if dat.GetAt(i).Structure="listnode" set prp=dat.GetAt(i) quit
	
		if prp {
			if %compiledclass.ClassType="serial" { set list="", i=1 } else { set list=$lb(""), i=2 }
			for i=i:1:prp.Values.Count() set list=list_$lb(prp.Values.GetAt(i).Value)
		}
		
		do %code.WriteLine(" if name="""" quit """_$lts(list)_"""")
		do %code.WriteLine(" quit $lf($lfs("""_$lts(list)_"""),name)")
	}
	
	if list="" write !,"*** No properties found! ***"
	quit $$$OK
}

}

Two test classes

Class DC.TestPerson Extends (%Persistent, DC.PropInfo)
{
Property Name As %String;
Property Addr As DC.Address;
Property Salary As %Numeric;
Property Expertise As list Of %String;
}


Class DC.Address Extends (%SerialObject,DC.PropInfo)
{
Property Street As %String;
Property City As %String;
}

Now you can do things like:

write ##class(DC.TestPerson).PropInfo()         --> ,Name,Addr,Salary,Expertise
write ##class(DC.TestPerson).PropInfo("Salary") --> 4
// you can use the slot number for direct data access:
write $list(^DC.TestPersonD(id),slotnumber)  gives you the same value as
write ##class(DC.TestPerson).SalaryGetStored(id)

// the same game for serial classes
write ##class(DC.Address).PropInfo()       --> Street,City
write ##class(DC.Address).PropInfo("City") --> 2

// in case you have an instance
set pers=##class(DC.TestPerson).%OpenId(id)
write pers.PropInfo()      --> ,Name,Addr,Salary,Expertise
write pers.Addr.PropInfo() --> Street,City

// etc.

Another reason for the <CLASS or ROUTINE DOES NOT EXISTS> message is that you're in the wrong namespace. Your installation has four namespaces by default: %SYS, DOCBOOK, SAMPLES, and USER. Another possibility could be misleading YouTube videos that use classes from newer Cache or IRIS versions without notice.

If you want to work with class parameters, you can choose between two basic variants

Parameter GlobalName1 = "^Global";
Parameter GlobalName2 = {$name(^Global)};

ClassMethod Test()
{
	set @$name(@..#GlobalName1)@("index")="abc"	// using variante 1
	write @..#GlobalName2@("index")				// using variante 2
}

If you have set the "handling of undefined" settings to 2 then in either case (you set x to a nullstring OR x is undefined) in statements like

if $d(@x@(whatever))
set d=$d(@x@(whatever))

you get a <SYNTAX> error, because nullstring (the value of x) is not a valid name for a local or global variable.

In a terminal session issue the following commands and check each output with mine:

kill         // kill all local variables
write x=""   // the output is 1 (because of your setting)
if $d(@x)    // the output is a <SYNTAX> because "" is not a valid name
set d=$d(@x) // the output is a <SYNATX> because "" not a valid name

If your output is different from the above, than please contact the WRC
My $ZV is IRIS for UNIX (Ubuntu Server LTS for x86-64) 2021.2 (Build 649U) Thu Jan 20 2022 08:49:51 EST

I keep on nagging...
I expected to see the environment of your IF-line: is it in a block structure or not. Which variables are initialized, etc. But you always give me the same expressiveness information: just the IF-Statement.

Nevertheless, I think, I know, why you get the <SYNTAX> error:

- under "default conditions", executing the following lines gives you an <UNDEFINED> error

  kill
  set a=1,b=2   // x is not defined
  if $d(@x@(a,b))

- but the very same lines gives you a <SYNTAX> error if the setting, how undefined variables should be handled is set to 1 or 2 (I suppose, in your case it is set to 2) because x as nullstring is not a valid variable or global name.

To see the setting in question, go to:
ManagementPortal-->SystemAdministration-->AdditionalSettings-->Compatibility and look for the setting "Undefined". I'm pretty sure, there is a 2 (or maybe 1). The default setting is 0.

Hint: the above mentioned setting is a per job setting, you must start a new job to have the new setting in effect

I think you are leaving out some important information: you have a terminal session, but you are calling either a routine (.mac) or a (class) method, since a standard terminal session does not accept multi-lines (as you wrote above).
And when you call a routine or method, it would be nice to see that function/method. By the way, for indirection to work, you need variables with global visibility. So I'm assuming you're mixing some existing variables from your terminal session with variables from your routine/method.