Use whatever is easiest. Your time is far more valuable than computer time and honestly a GUI is not a real-time operation. If you shave off a few microseconds in your string handling no one could even observe it.
Maybe I'm just getting too old. Maybe I've seen the debate too many times. I remember spending too much time worrying about "bloat". I worked with another developer who refused to use virtual functions because they were "too bloated." Now my viewpoint is, I had these same questions back in the 90s when I had a machine with 32Mb of RAM and 100mhz CPU now I have 8192Mb of RAM and 4 3500mhz CPUs. If the worry wasn't relevant then, it certainly isn't relevant now, at least for the software we write. Plus, I write code in Java, so I swim in the bloatiest of bloated code, and I'm still doing fine.
The rules of optimization haven't changed: Don't optimize prematurely. Complexity (O(n) vs O(n^2)) blows away any other form of optimization. A quicksort in JS on your phone will destroy your C++ bubble sort on your core i7.
I do add one caveat. When picking a framework technology, don't be stupid. This is an exception to the optimize prematurely rule. I did poke fun at JS and C earlier, but using something like JS for example puts limits on the maximum performance you can achieve. So, if you think you'll get to the point where you must compete on performance, don't make a choice that forces a rewrite. But, for personal/hobby software, you'll never run into that. For most enterprise or line-of-business/professional software, even then, you won't run into that limit. Games and media (codecs) are where it matters. So you're in the game field, but don't fool yourself into thinking you have a team of 30 devs to work on figuring out a new way to do strings...