What causes lag and how can I resolve it?

Lag is quite often part of my life. I find it happens most often when I’m working in Gmail. I have to use Gmail, because that’s where my work email is housed. When I’m afflicted, it’s like typing through mud. I have to wait for my computer to catch up with me. I type very fast, but when the ‘lag’ hits, I can’t even type slowly. It really slows down my work progress.

I’ve tried clearing my cache. I’ve turned off apps that might slow things down – even ones I’d like to use like spellcheck and editing apps. I tend to use Chrome when doing my mail because I do prefer to use one app that’s only available through Chrome. I’ve shut down everything but Chrome. Lag is still there dragging me down. I’ve rebooted my computer – repeatedly.

What might cause this? Is there anything else I can do about it?
It seriously drives me nuts.