Lately, I've done a number of optimizations on the Ruby on Rails server I manage:

• Set Ruby's GC parameters to run GC less frequently (~50% boost)
• Using jemalloc as the memory allocator (~20% boost)
• Using gctools gem for out-of-band GC run (~50% less time in GC during requests)
• And some gems to monkey patch slow methods: fast_blank (~5% boost). I've also tried escape_utils but it didn't help.

After reading many positive benchmarks on Phoronix I decided to use the new promising compiler, Clang. First I did some tests on my Ubuntu 14.10, which ships with GCC 4.9. Clang 3.5 made ruby 2.1.5 8% faster! But when compared to the Debian Wheezy's GCC version (4.7.2), ruby 2.1.5 became 44% faster! Another interesting boost :) Finally we have a very interesting alternative for GCC.