Plug-ins can consume large amounts of CPU resources, and it's is a good idea to use as few instances of each plug-in as possible. Reverbs can be inserted to auxiliary tracks, and audio can be bussed to these tracks from multiple sources. Similarly, multiple MIDI tracks can send to a single virtual instrument. Both methods conserve resources by loading the plug-in, and thus the CPU load, only once. Limiting the number of active plug-ins has the added benefit of keeping your session smaller and more streamlined.

To monitor how your computer is utilizing its resources, right-click in an empty space on the bottom task bar (somewhere between the Start button and the clock). Select Task Manager. The Performance tab will give you a fairly accurate idea of the load put on your CPU. This meter takes into account everything that is running. Keep in mind that it can be a little jerky; what you're looking for is an average measurement over several seconds. Try to keep the processor below an upper limit of 65-75%. Higher loads than this cause dropouts, freezes, and crashes. If the load is too high, you can remove plug-ins or applications. If this still doesn't help, then the solution very well could be to increase the amount of RAM installed in your computer.