Don't convert hash keys, it's slower than using the "natural" key of the input data

run

$ ruby -v t.rb
ruby 2.4.2p198 (2017-09-14 revision 59899) [x86_64-darwin17]
Lookup
Warming up --------------------------------------
Symbol Hash, Symbol Key
                       259.197k i/100ms
String Hash, String Key
                       231.953k i/100ms
Symbol Hash, String Key
                       203.959k i/100ms
String Hash, Symbol Key
                       197.515k i/100ms
Calculating -------------------------------------
Symbol Hash, Symbol Key
                          6.998M (± 2.9%) i/s -     34.992M in   5.004735s
String Hash, String Key
                          5.407M (± 2.4%) i/s -     27.139M in   5.021924s
Symbol Hash, String Key
                          4.018M (± 1.5%) i/s -     20.192M in   5.026624s
String Hash, Symbol Key
                          3.788M (± 2.2%) i/s -     18.961M in   5.008168s

Comparison:
Symbol Hash, Symbol Key:  6997934.0 i/s
String Hash, String Key:  5407394.5 i/s - 1.29x  slower
Symbol Hash, String Key:  4017928.8 i/s - 1.74x  slower
String Hash, Symbol Key:  3788016.8 i/s - 1.85x  slower

test.rb

require "benchmark/ips"

STRING_KEYS = (1..1000).map{|x| "key_#{x}"}.shuffle
FROZEN_KEYS = STRING_KEYS.map{|x| "fr_#{x}".freeze}
SYMBOL_KEYS = STRING_KEYS.map(&:to_sym)

# If we use static values for Hash, speed improves even more.
def symbol_hash
  SYMBOL_KEYS.collect { |k| [ k, rand(1..100)]}.to_h
end

def string_hash
  STRING_KEYS.collect { |k| [ k, rand(1..100)]}.to_h
end

# See this article for the discussion of using frozen strings instead of symbols
# http://blog.arkency.com/could-we-drop-symbols-from-ruby/
def frozen_hash
  FROZEN_KEYS.collect { |k| [ k, rand(1..100)]}.to_h
end

SYMBOL_HASH = symbol_hash
STRING_HASH = string_hash
FROZEN_HASH = frozen_hash

def lookup_symbol_hash_from_string
  SYMBOL_HASH[STRING_KEYS.sample.to_sym]
end

def lookup_string_hash_from_symbol
  STRING_HASH[SYMBOL_KEYS.sample.to_s]
end

def lookup_string_hash_from_string
  STRING_HASH[STRING_KEYS.sample]
end

def lookup_symbol_hash_from_symbol
  SYMBOL_HASH[SYMBOL_KEYS.sample]
end

Benchmark.ips do |x|
  puts "Lookup"
  x.report("Symbol Hash, Symbol Key") { lookup_symbol_hash_from_symbol }
  x.report("String Hash, String Key") { lookup_string_hash_from_string }
  x.report("Symbol Hash, String Key") { lookup_symbol_hash_from_string }
  x.report("String Hash, Symbol Key") { lookup_string_hash_from_symbol }
  x.compare!
end