eval does not load another interpreter nor does it create another process.

eval is slower than most alternatives so you try to avoid doing eval inside of a loop or an oft-used subroutine. But eval isn't so slow that you should be too afraid to use it. But there are also plenty of reasons other than performance to not use eval most of the time.

Back to the question, the speed of " vs. ' inside an eval would be nearly impossible to detect, much less being something to worry about. After all, ' still has to check for \\ and \'.