results.txt

$ ruby -v
ruby 2.7.1p83 (2020-03-31 revision a0c7c23c9c) [x86_64-darwin19]
$ ruby /tmp/struct.rb
Warming up --------------------------------------
                hash     1.450M i/100ms
               naive     8.267k i/100ms
         lazy-cached    68.069k i/100ms
            explicit   415.985k i/100ms
Calculating -------------------------------------
                hash     14.223M (± 3.4%) i/s -     71.047M in   5.001074s
               naive     84.675k (± 3.0%) i/s -    429.884k in   5.081532s
         lazy-cached    683.427k (± 1.8%) i/s -      3.472M in   5.081364s
            explicit      4.212M (± 2.5%) i/s -     21.215M in   5.040363s

Comparison:
                hash: 14222972.0 i/s
            explicit:  4211882.2 i/s - 3.38x  (± 0.00) slower
         lazy-cached:   683426.7 i/s - 20.81x  (± 0.00) slower
               naive:    84674.5 i/s - 167.97x  (± 0.00) slower

sizes.rb

require 'objspace'

def NaiveStruct(**kwargs)
  Struct.new(*kwargs.keys).new(*kwargs.values)
end

ExplicitStruct = Struct.new(:foo, :bar)

def print_size(label, *elements)
  puts "#{label}: #{elements.map { |e| ObjectSpace.memsize_of(e) }.sum}B"
end

print_size("hash", {foo: 1, bar: 2})
print_size("explicit", ExplicitStruct.new(foo: 1, bar: 2))
print_size("naive", NaiveStruct(foo: 1, bar: 2), NaiveStruct(foo: 1, bar: 2).class)

sizes.txt

hash: 168B
explicit: 40B
naive: 632B

struct-bench.rb

require 'benchmark/ips'

def NaiveStruct(**kwargs)
  Struct.new(*kwargs.keys).new(*kwargs.values)
end

STRUCT_CACHE = {}
def LazyCached(**kwargs)
  keys = kwargs.keys.sort
  klass = (STRUCT_CACHE[keys] ||= Struct.new(*keys, keyword_init: true))
  klass.new(**kwargs)
end

ExplicitStruct = Struct.new(:foo, :bar)

Benchmark.ips do |x|
  x.report('hash') { {foo: 1, bar: 2} }
  x.report('naive') { NaiveStruct(foo: 1, bar: 2) }
  x.report('lazy-cached') { LazyCached(foo: 1, bar: 2) }
  x.report('explicit') { ExplicitStruct.new(foo: 1, bar: 2) }
  x.compare!
end