Physically, a Fusion drive is 2 drives: an SSD and an HDD. OS X treats them as one drive but puts all your most frequently-accessed files on the SSD. SSDs are MUCH faster than HDDs, so for the most part, you will see much less lag related to accessing files, booting up, etc. I have a 2010 iMac that I added an SSD to in place of the DVD drive and still have a larger HDD in place. I can tell you I'll never go back to an HDD-only setup. It is just sick how much faster apps seem to pop now. Anything CPU-intensive will not be faster, but any lags you may experience from data being written to a drive and read from the drive will be dramatically reduced.