Increasing buffer size...
The processor(s) in your computer, while very fast, still need a certain amount of time to process everything you have going on in your projects. Now, the following is an over-simplification, but I think it'll get the point across:
Did you ever watch the I Love Lucy episode where she's working in the chocolate factory? Go to this link and start watching from about 4:04...
So now, the less chocolaty version! The I/O buffer values are in samples. So with that in mind...
One stereo audio track with an EQ on it is going to require a certain amount of time to process raw audio (as it lives on your hard drive) and play it back processed with the EQ. For argument's sake, let's say this takes 10 sample's worth of time to accomplish. If your buffer size was set to 32 then you'd probably be fine.
Now you have, say, 10 stereo tracks, each with an EQ. So now your computer needs at least 100 samples worth of time to read all the data from your hard drive, process it, and spit it all back out. So you probably need a buffer size of 128, at least, to prevent the system from reporting that it can't process all the data in time ("overload").
Having the buffer set too high can lead to problems too, but the general rule of thumb is that you want to set your buffer size as small as possible without sacrificing the performance of your system.
Again, this is an oversimplification, but that's the theory.