bm_cachename.rb

#!/usr/local/bin/ruby
require 'benchmark/ips'
require 'base64'
require 'digest'

N=10_000
data = Digest::SHA256.digest("")
Benchmark.ips do |x|
  x.report("base64") do
    N.times do
      Base64.encode64(data)
    end
  end
  x.report("urlsafe_base64") do
    N.times do
      Base64.urlsafe_encode64(data)
    end
  end
  x.report("to_i(32)") do
    N.times do
      data.unpack("H*")[0].to_i(16).to_s(32)
    end
  end
  x.compare!
end

result

Warming up --------------------------------------
              base64    22.000  i/100ms
      urlsafe_base64     8.000  i/100ms
            to_i(32)     6.000  i/100ms
Calculating -------------------------------------
              base64    223.391  (± 5.8%) i/s -      1.122k
      urlsafe_base64     84.366  (± 8.3%) i/s -    424.000 
            to_i(32)     61.936  (± 6.5%) i/s -    312.000 

Comparison:
              base64:      223.4 i/s
      urlsafe_base64:       84.4 i/s - 2.65x slower
            to_i(32):       61.9 i/s - 3.61x slower