If you have developers who for one reason or another prefer a given platform, then the most important performance comparisons are about how close various frameworks on that platform get to the performance of the platform itself.
Knowing how much I'm giving up in performance in order to get the features a given framework gives me is an important consideration. Also understanding when it's worthwhile to work outside the framework on the bare platform given the speedup I'll get versus the cost I'll incur by doing so is a very important optimization decision-making tool.
There are performance numbers for a framework (Cake PHP, for example) and for the raw primitives of the platform it runs on (PHP, in that case). By finding the ratio between the two one can arrive at the performance loss attributable primarily to the framework you've chosen.
See my "framework optimization index" in comments below for a rundown on all these ratios which I was able to back out from this set of benchmarks.
Knowing how much I'm giving up in performance in order to get the features a given framework gives me is an important consideration. Also understanding when it's worthwhile to work outside the framework on the bare platform given the speedup I'll get versus the cost I'll incur by doing so is a very important optimization decision-making tool.