Actually I wasn't considering this but you and many others in here pointed out mass firing would be very bad with very good reasons. Now I changed my mind about this, even if I finalize my decision on firing it'll be worst to better in every 2 months (unless they change in the middle).
I actually think dragging it out over several months would be even worse. I wouldn't fire more than 2 that way. Your good employees will be wondering if they are next and if they should be polishing their resume (which they'll do instead of work you need done)