If you're using Windows 7 (or Vista, I believe) then click Start, type "Resource Monitor" into the search box, and hit <Enter>. Click on the Disk tab.
Also monitor CPU usage by opening Task Manager (Shift + Ctrl + Esc) and selecting the "Performance" tab.
Watch the Resource Monitor disk "queue length" graph(s) and the Task Manager CPU graphs as you're doing an editing task which seems laggy. If the CPU is solidly at 100% then you have a CPU bottleneck. If one or more disk graphs show a solid queue length of 1 or more then you have a disk bottleneck.