Just because IBM stuff is legacy doesnot means it is slow. Almost nobody is moving from a commercial database (Db2 for z/OS) to PostgreSQL because of performance. It is all to do with huge licensing cost. Talk to any veterans nothing matches the speed and stability of mainframe. search about z16 (tell me anything that guarantees 9 nines of availability)

Eh DB2 is still a pretty good database all things considered. At the end of the day it's pretty much exactly like every other modern SQL database, and it's relatively polished given its been around since the beginning.

As a programmer at a bank, we are heavy into DB2 but I also get to use plenty of more trendy databases, and I think if you got rid of branding and blindfolded people or whatever, the vast majority of backend programmers wouldn't be able to tell them apart or pick db2 out of a lineup.