[LAU] KMS and xruns

I was overjoyed after my last attempt at running the latest intel driver and setting up KMS for it, only to later find out that this had a high price. The xruns are horribly too many, and events that trigger them are window minimise/restore/maximise, disabling/enabling compositing, even changing tabs in a window! Now I'm starting KMS before even my initscripts run, very early in userspace, and I've tried running it later as well - to no avail. I'm back to a regular environment without this KMS thing, partly content (that I'm safe again) and partly disappointed (I thought I almost had the perfect system). Now I would really like to know from someone in-the-know why and how KMS are underruns are related. I'm sure this is not intel-specific.