It really depends on what your bottleneck is. If your bottle neck is already CPU, then TLS is not that big of a deal. If, OTOH, your bottleneck is disk, then implementing TLS can create substantial overhead.
TLS adds quite a bit of memory overhead, it increases the network bandwidth due to padding, and if your CPU is eqipped with AES extensions (modern ones) then you still do compression usually.
it can be difficult to cache certain things with TLS also- since you can't do transparent caching. (although this is more a problem for those running squid proxies at work on a stretched out line).
TLS is certainly an overhead and it's not required in cases where I'm checking a bulletin about earthquakes.