Well, maybe not so fantastic! I didn't look at CPU utilization until you asked the question, but it went from about 17% to 32%! I changed it back to 0 seconds, and the CPU utilization went back down to 17%.
FWIW, I have 7 cameras and had set the pre-trigger buffering on 3 of them.
P.S., I checked the help file and found this:
Please note that the use of this feature consumes considerable memory and some CPU resources--it should only be used if you have both to spare. In fact, it may be impractical if not impossible to use more than 0.5-1.0 seconds here unless you are also using Direct-to-Disc recording (see below).
I had 3 cameras set to a 2-second buffer - I removed the buffer from all but one camera and set it to a .5 second buffer, and it's only using an extra 3% CPU now, which is acceptable.