テストしますた

string = 'test' * A; symbol = string.to_sym
h = Hash.new
#Hashの中に適当に代入

h.clear
starttime = Time.now
B.times do h[string] = string end
endtime = Time.now
p endtime - starttime

h.clear
starttime = Time.now
B.times do h[symbol] = symbol end
endtime = Time.now
p endtime - starttime


A = 100(400文字); B = 1000000
String: 6.625
Symbol: 0.625

A = 10(40文字); B = 10000000
String: 15.938
Symbol: 6.265

長い文字列は効くなぁ