This IDKEYOpen looks as capricious beast. Because of some reasons digi.packet has no such method. But its sibling class do have it although in lowercase:

zidkeyOpen(K1,K2,concurrency=-1,sc) public { Set:'($data(sc)#2) sc=1
Quit ..%OpenId(K1_"||"_K2,concurrency,.sc) }

A few other classed  have IDKEYOpen in upper case:

zIDKEYOpen(K1,concurrency=-1,sc) public { Set:'($data(sc)#2) sc=1
Quit ..%OpenId(.K1,concurrency,.sc) }

Looks as the compiler generates this method only in some cases. But why in lower case? Strange.

What for the performance, the method looks as a very simple wrapper around %OpenId, so no much performance difference to expect.