When you are just using a CPU, not at all. It runs as the lowest priority in windows so you really dont see lag. I run multiple i3-i7 and i have no problems, even on a cheap 40$ CPU my buddy has it tends to not hurt, you can always right click the icon in task bar and set it to "low, medium, high" while you work, or even check "pause" then unpause when you are done.

A GPU however does make the screen lag while running it high and working on it at the same time. I have a R9 290x that i set to "medium" when i work on it and "low" when i play Starcraft II on it, then back to "high" when im done.