I guess it's from experience, that when there's a bottleneck in my work and in many other areas I've seen it's down to lack of ram hitting VM.

Now lack of CPU is less crippling in a way, the CPU just divides n ways and things run proportionately slower. If paging happens, it can easily be far worse than lack of CPU as disk is so slow (ok, SSDs these days may ameliorate that; I've no experience there).

So why? Experience suggests lack of ram is more common than lack of cpu and the effect is worse. Of course the best thing to do is examine your system before you take anyone's advice, including mine.