Well, you will need both if you want to play with RaceRoom. (But having a CPU with good single core performance is more important than a top level GPU, you should be fine with a mid-range graphics card).
I have a FX8320E, which is poor in single core performance... It suffers with more than 15 cars on track... And my 1070 is totally bottle-necked here for sure..
This is going to sound kinda whacked, but if you are noticing unreasonably poor performance, try changing the full screen/windowed mode setting (even if you run triples) At one time raceroom was driving me mad with poor performance, and switching this made a monumental difference. This is a very specific case of course, but worth noting on the off chance anyone else suffering from this...a few of us were a year or so back.