Are you my father?

bench.rb

require 'benchmark/ips'
require 'active_support/inflector'
require 'set'

class A; end
class B < A; end

klasses = [Struct, A].freeze
strings = ['Struct', 'A'].freeze
set = Set.new(strings)

Benchmark.ips do |x|
  x.report('klass') { klasses.none? { |k| B <= k } }
  x.report('const') { strings.none? { |k| B <= k.constantize } }
  x.report('strings') { (strings & B.ancestors.map(&:name)).empty? }
  x.report('set') { B.ancestors.none? { |a| set.include? a.name } } 
end

#  Calculating -------------------------------------
#                 klass     76805 i/100ms
#                 const     23443 i/100ms
#               strings     23379 i/100ms
#                   set     41934 i/100ms
#  -------------------------------------------------
#                 klass  1906953.4 (±3.5%) i/s -    9523820 in   5.000878s
#                 const   362407.1 (±3.3%) i/s -    1828554 in   5.051450s
#               strings   331800.5 (±3.6%) i/s -    1659909 in   5.009901s
#                   set   733412.7 (±3.4%) i/s -    3690192 in   5.037502s