I have been having a similar problem for the past week or so. My MacBook Air becomes incredibly unresponsive -- I can't type, I can't open windows, I can't open task manager without waiting for tens of seconds before something happens. In addition, my mac has restarted itself at least once. Prior to this, the MacBook Air worked flawlessly.
After investigating what was happening, I discovered that com.process.avast.(sorry I can't remember which) was using up 100% of the CPU cycles doing something. I want to say it was the filesystem, but I am not positive it was. I can say, that a com.avast.X process was bringing my machine to its knees and made it unusable. Because I needed to use my computer, I kept killing the com.avast.X process and I would get about 30 seconds of time before it re-started itself and hogged all of the CPU again.
The problem comes and goes and I don't know what is causing it.
I am on an MBA running 10.9.5 and Avast 10.2.