So I'm about to get into building a large scale browser-based app testing solution, but the testing I'm doing isn't one of correctness, but of performance. This

So I'm about to get into building a large scale browser-based app testing solution, but the testing I'm doing isn't one of correctness, but of performance. This is a particularly persnickety problem since I need a solution where the probing itself doesn't introduce side effects that invalidate the test results. For example, many of the solutions I've seen use iFrames to isolate the test environment, but that introduces an entirely new and significant layer of redirection, especially when your results rely on GPU-related functions like CSS transforms. Can anyone who has done lots of browser based testing shed light on what solutions out there introduce the fewest or no side effects when probing?