The problem with lag in KitKat is due to a problem in the code (related to some EGL contexts not being destroyed correctly), but it's a problem that can be solved by the community. Judging from the comments left in the source code, there were some issues with the eglDestroyContext function that weren't fixed properly. It's quite possible that Broadcom's engineers never managed to solve the problem, especially since there are no BCM21553 devices officially running KitKat, where the bug seems to manifest more frequently.
The most I have been able to do about this problem so far is to prevent crashes when the eglDestroyContext problem manifests. In previous builds, you would see random app crashes when the bug started happening. In more recent builds the crashes won't occur when the bug manifests - instead, there will be a performance loss and lag when opening/closing apps. Once the bug starts happening, rebooting is the only way to clear the bug and regain full performance again.
Nobody has been helping me to troubleshoot this problem (and by that, I mean, downloading the source and trying to solve the issue in the code properly). That's the real problem here.