Ruby 2.5 added lazy proc allocation for block parameters

irb> def greet
irb>   yield
irb> end
  => :greet
irb>
irb> def greet_with_welcome(&block)
irb>   puts 'Welcome'
irb>   greet(&block)
irb> end
  => :greet_with_welcome
irb> greet_with_welcome { p 'BigBinary' }
Welcome
"BigBinary"
  => "BigBinary"

In Ruby 2.4 when we pass a block to a method, which further passes to another method, Ruby creates a new Proc object by the given block before passing this proc to the another method.

This creates unnecessary objects even when the block parameter is not accessed. It also creates a chain of Proc objects when the block parameter is passed through various methods.

Proc creation is one a heavyweight operation because we need to store all local variables (represented by Env objects in MRI internal) in the heap.

Ruby 2.5 introduced a lazy proc allocation. Ruby 2.5 will not create a Proc object when passing a block to another method. Insead, it will pass the block information. If the block is accessed somewhere else, then it creates a Proc object by the given block.

This results in lesser memory allocation and faster execution.

Ruby 2.4

irb> require 'benchmark'
  => true
irb> def greet
irb>   yield
irb> end
  => :greet
irb>
irb> def greet_with_welcome(&block)
irb>   puts 'Welcome'
irb>   greet(&block)
irb> end
  => :greet_with_welcome
irb>
irb> Benchmark.measure { 1000.times { greet_with_welcome { 'BigBinary' } } }
Welcome
Welcome
...
...
...
  => #<Benchmark::Tms:0x007fe6ab929de0 @label="", @real=0.022295999999187188, @cstime=0.0, @cutime=0.0, @stime=0.01, @utime=0.0, @total=0.01>

Ruby 2.5

irb> require 'benchmark'
  => true
irb> def greet
irb>   yield
irb> end
  => :greet
irb>
irb> def greet_with_welcome(&block)
irb>   puts 'Welcome'
irb>   greet(&block)
irb> end
  => :greet_with_welcome
irb>
  irb> Benchmark.measure { 1000.times { greet_with_welcome { 'BigBinary' } } }
Welcome
Welcome
...
...
...
  => #<Benchmark::Tms:0x00007fa4400871b8 @label="", @real=0.004612999997334555, @cstime=0.0, @cutime=0.0, @stime=0.001524000000000001, @utime=0.0030690000000000023, @total=0.004593000000000003>

As we can see, there is considerable improvement in execution time when a block param is passed in Ruby 2.5.

Here is the relevant commit and discussion.