I often need to browse folders full of STLs, and I was dissatisfied with the performance of most programs. So I wrote one myself where you can quickly skip to the next file with right arrow key.
If you’re a 64-bit Windows user, you can download it at http://papas-best.com/stlviewer_en
I hope it’s as useful to you as it is to me. If not, drop me a note and I’ll try to fix things!