As above, the most likely cause is insufficient RAM to hold all of your active applications in memory. Once you have swapping and frequent re-loading from storage due to low RAM (relative to what you are doing), performance takes a dump.
When I upgraded from my 8GB Core2Duo because the stuff I was doing on it ate so much RAM that I had to spread my software and swapfile across three HDDs to keep performance tolerable to an i5 with 16GB of RAM, I quickly discovered that even 16GB of RAM was still not enough to completely eliminate swapfile lag and upgraded to 32GB.