However I see a few comments that missed a bit the point, and this is probably my fault because I did not specified some very important thing:
Once you substitute not just an hello world template, but a few templates N times (since you have N comments in the page), the performance starts to be so slow you can't serve more than about 10 requests per second, just because template substitution is so damn slow if you don't preload templates, and the obvious way to do it is calling :erb (you see this in all the Sinatra code around, in the examples, and so forth).
10 requests per second instead of the 1000 you get even with 'N' includes if you perform a faster substitution, or if you preload the templates, or using any other trick, is a huge difference.