You wrote

BUT the server is 6 times faster if OpenId replaced with simple read of a large global (s p=^someLargeGlobal). Any ideas what makes OpenId so slow only on the server?

while not specifically answering your general question, note that opening an object is very different than 

Set p=^SomeLargeGlobal.

When you call %OpenId

1. Concurrency is checked.  While not widely used there is a second parameter to the %OpenId method that controls the behavior of concurrency  and you may want to consider providing the desired concurrency option.  In the past if I was not concerned with concurrency as I just wanted to read data I would use option 0.
2. Each and every property defined in your class is loaded into memory, ie the data is read from the global and then the individual properties are set accordingly.  Depending on how complex your class is the difference between 

Set p=^SomeLargeGlobal 

and %OpenId can be quite different, at least academically.

If your class as relationships and/or serial/embedded classes I do not believe those properties are fully loaded into memory, but I could be incorrect.

In practice, if I need an object I use objects, if I need only a small part of the object/row I'll use embedded SQL for better performance.

Again this does not specifically answer your general question but I think it is useful to understand what %OpenId does and why it's not the same as 

Set p=^SomeLargeGlobal.

with your updated problem description where your globals only contain a single node I would consider the serial/embedded approach.  This still don't address your request for a variable name for the storage, but it does mean for the columns/properties you can define them in the serial class once and then embed in the class that represents global a and global b.  It also means in the storage map you just have to define the data node as the serial property. 

Let's say we could get it to work such that the storage map is based on a variable, I suspect other things wouldn't work.  For example, in Documatic when you view a class you can select the Storage checkbox and get a display that looks like 

I suspect if somehow you can get the storage map to be dynamic and based on a variable name this display would fail or not show you the value of the variable.

Depending on what you exactly mean by the same structure you consider using a Serial Class definition that is embedded in both of your classes.  However, if the structure is stored across several global nodes I do not think you could use this as the Serial class would define the pieces(whether delimited or $ListBuild pieces) and then the serial property is described in your 2 classes to occupy a single node.

You could also consider defining a single abstract class that describes the properties and have your 2 classes inherit from your abstract class.

With regards to having a variable defining the data location I suspect that it may not be doable, even if it were I don't see how it improves things in a significant way.

I would be interested in the English version as well.

I'm generally looking at the query plans either from the SMP or from the context menu in Studio while writing class queries/embedded sql statements.

One issue that I've seen is that while the query plan is very good, an in many cases better than what other dbms's provide, when a sub-query is part of your query statement it's not exactly clear when it is utilized in the query plan.  For example, I have this query plan

I cannot tell with 100% certainty where "subquery" is called.

While the cost may be high, the real question might be better focused on the performance.  Relative cost is just that, relative. 

What does the rest of the query look like?

What is the time to first row, time for all of the data? 

Does the query plan utilize the index on the StartDate column?

A couple ideas

If it's for debugging I use $System .OBJ.Dump(oRef)

If it's in application code I ran across something that looked like

While prop '= "" {
//  Use global lookup instead of %Dictionary.Class query so users don't need privileges on that table
Set tPropClass = $$$comMemberKeyGet(sourceClass,$$$cCLASSproperty,prop,$$$cPROPtype)
 

If you have more recent versions of Cache you will likely benefit by using %PARALLEL especially if you have a large number of cores for your environment.

Might it be simpler to just use the sql function GREATEST.

While not specifically an answer to your question I have used Class Projections  as a way to detect when a class is compiled or removed.  This will not allow you to see the changes between classes but could be useful to see when classes are compiled.

Given that you guys have opened the hood, while not specifically a DOCBOOK request, I would like to see https://www.chromium.org/tab-to-search be supported for DOCBBOOK content.  As an example I'd like to when using Google Chrome be able to type 

docs.intersystems.com [tab] {SearchTerm} 

 and have the page respond with a list of results based on {SearchTerm}

I understand there may be issues with what version of DOCBOOK to show but it might be useful to just show results from the latest version.

Just checking as I still see Technical Articles I assume if you add content to DOCBOOK this additional content will still be part of this new UI?

One aspect that has been lost in the transition is to have the Search Dialog always on the page.  If you go to this page as an example http://docs.intersystems.com/beta/csp/docbook/DocBook.UI.FramePage.cls?K... in order to initiate a new search it appears I have to

1. Select this back button on the top left hand corner 
2. Scroll the left hand side menu, list of books to the very top
3. Place my cursor in the search field and enter a new search term.

Whereas in the old system I could 

1. Press the [Home] key to be taken to the top
2. Enter a new search term.

I would vote to make the search box visible at all times.

If you can share the Show Plan information for each query that will probably add some insight.   Given that the second query has only one column it may be that there are additional adjustments to the query that would yield better performance, although this does not directly address your specific question as to why it is much slower.  At the same time a query that is taking well over 30 minutes seems like there is something not quite right.

Great news.

only comment I would add is I would think the peformance chart in this article would be easier to quickly digest if all of the values were in ms.