bad code/non-performance optimized can be pain in both. a lot of games done in unity have quite a bit of 'sluggishness', that's opinion from some of my friends whom are those 8h per day play, 6 hours of sleep gamers. so if youre after high performance & amazing looks which is even more important in vr AND u are/have decent devs then ue is a better choice (personal opinion). comparing the speed c# vs c++... well, there's no no point right? ;)