Servers are much cheaper than humans.

First of all, the amount of time that'd be required to rewrite things in said "higher performance" language with the hopes of getting some improvements would be very costly. It's not something that'd be done in even a month by one person. It'd be a whole company effort. Learning curve of new language, etc.

With that aside, I guarantee, for our use case at least, I'd rather invest my time into tuning a Varnish config and save the same # of servers without all the man power wasted.

Our server costs are much much cheaper than our human costs.