Grab an app from play store to monitor what's using the CPU. Something like Watchdog Lite or Usage Timelines Free. They'll show you what's hogging your processor and causing lags. Watchdog can also alert you for apps using X amount of cpu.
In my experience android system uses a fair bit of CPU on initial boot but after that this ROM idles nicely - even with Google sync plus my email (k-9).
Watch out for the Facebook app and also stock email with exchange sync. These killed my phone so are now removed. I use mobile browser for Facebook and Enhanced Email for exchange and have no day to day CPU issues. Best ROM I've used so I'm sticking with it!
Top notch! Working like a charm now. Thanks a million :-) .
Ps. For those of you posting about random reboots I can confirm that officially unlocking the bootloader will sort this issue. Well, it did for me anyway! Search the forum for g300 official unlock (as I don't have the link to hand sorry).