Quick test

benchmark_methods.rb

$: << File.expand_path("../../lib", __FILE__)

require 'database_cleaner'
require 'mongoid'
require 'mongoid-rspec'
require 'mongoid_token'
require 'benchmark'

Mongoid.configure do |config|
  config.master = Mongo::Connection.new.db("mongoid_token_benchmark")
end

DatabaseCleaner.strategy = :truncation

# start benchmarks

@token_length = 32
@times = 10000

class Link
  include Mongoid::Document
  include Mongoid::Token
  token :length => @token_length, :contains => :alphanumeric
end

Link.destroy_all
puts "-- Alphanumeric token of length #{@token_length} (#{62**@token_length} possible tokens)"
Benchmark.bm do |b|
  b.report("#{@times} records old   "){ @times.times{ Link.new.send(:generate_token, @token_length, :alphanumeric) } }

  def Link.generate_token(size, characters)
    (1..size).collect { (i = Kernel.rand(62); i += ((i < 10) ? 48 : ((i < 36) ? 55 : 61 ))).chr }.join
  end
  b.report("#{@times} records kernel"){ @times.times{ Link.new.send(:generate_token, @token_length, :alphanumeric) } }

  def Link.generate_token(size, characters)
    chars = ('a'..'z').to_a + ('A'..'Z').to_a + (0..9).map(&:to_s)
    (0...size).collect { chars[Kernel.rand(chars.length)] }.join
  end
  b.report("#{@times} records chars "){ @times.times{ Link.new.send(:generate_token, @token_length, :alphanumeric) } }

  def Link.generate_token(size, characters)
    ActiveSupport::SecureRandom.base64(size*2).gsub(/(\/|\+|\-)/, '')[0..size-1]
  end
  b.report("#{@times} records base64"){ @times.times{ Link.new.send(:generate_token, @token_length, :alphanumeric) } }

end

results.txt

$ ruby spec/benchmark_methods.rb
-- Alphanumeric token of length 32 (2272657884496751345355241563627544170162852933518655225856 possible tokens)
      user     system      total        real
10000 records old     1.520000   0.010000   1.530000 (  1.544356)
10000 records kernel  1.510000   0.010000   1.520000 (  1.517105)
10000 records chars   1.550000   0.000000   1.550000 (  1.552146)
10000 records base64  1.550000   0.000000   1.550000 (  1.549400)