I think we're talking about an illusion here. There have been numerous posts about beta performance getting worse and the actual game being worse than beta.
I think it's a map issue, at least partly. I'm not talking about all of the individual cases, but map issues cover a big bunch of cases.
The first beta had three maps: fallen fighters, apartments. Both ran well - still do. Second set of maps introduced barracks and grain elevator. Both also ran quite nicely - still do. But the last set of maps introduced both troublesome maps: station and pavlovs. This leads to an illusion that the performance is getting worse.
And I know I missed a few maps, but they aren't significant anyways.