hash_access_benchmark.rb

require 'benchmark/ips'

h = { foo: :bar }

Benchmark.ips do |r|
  r.report('#[]')    { h[:foo] }
  r.report('#fetch') { h.fetch(:foo) }
end

output.txt

Calculating -------------------------------------
                 #[]     84463 i/100ms
              #fetch     58818 i/100ms
-------------------------------------------------
                 #[]  3004469.2 (±1.6%) i/s -   15034414 in   5.005338s
              #fetch  2196062.9 (±0.8%) i/s -   10998966 in   5.008783s

Looks like rb_hash_fetch_m winds up calling rb_scan_args and rb_block_given_p where rb_hash_aref does not. rb_block_given_p could move into the conditional which evaluates it if one doesn't care about the warning. I don't know what you could do about parsing the varargs though.

does not look like significanly slower anymore

~ $ ruby hash_access_benchmark.rb 
Calculating -------------------------------------
                 #[]   216.732k i/100ms
              #fetch   217.366k i/100ms
-------------------------------------------------
                 #[]     11.665M (± 1.7%) i/s -     58.301M
              #fetch     10.387M (± 0.6%) i/s -     51.950M
~ $ ruby -v
ruby 2.2.4p230 (2015-12-16 revision 53155) [x86_64-linux]