I tried flashing some of the higher kernels as well, but became too unstable at one point. But keep in mind I was putting heavy use on my phone, and had my overclocking app set to Maximum Performance", and running it at 1Ghz all the time couldn't have been good. Considering 1Ghz is how fast the stock processor is on my Bionic, and the phones were about 2 years apart from each other, that says at least something. This might be the key piece of information that I left out: Although in a post on another thread that I had flashed around 30 roms on my D1, I know that couldn't have been right because I was flashing new roms every 3-4 days. I was rooted for a year, that kinda tells you my phone went through a lot of software changes, maybe went through 50 or more roms. That, plus the hard usage on the processor, slowed it down.
Actually, with that much flashing, your problem likely had nada to do with clock speed. What started to kill mine (before the screen died) was my flash memory literally started to fail. It slowed the device down in day to day use simply because it was dealing with bad "blocks" of memory all the time. If i'd kept it up, my partitions would likely have developed enough bad blocks that I wouldn't have been able to flash it anymore.
Hard usage on a processor isn't going to slow it down. It's just going to up and die on you completely. Plus, if you were that close to the limits of the CPU, it would have become very unstable.