Why is Ruby's loop command slower than while true?

Ruby has a built-in loop command that executes the block following it forever (or until stopped by break). However, when comparing it against the functionally similar while true, it is significantly slower:

require "benchmark/ips"

NUMBER = 100_000_000

def fast
  index = 0
  while true
    break if index > NUMBER
    index += 1
  end
end

def slow
  index = 0
  loop do
    break if index > NUMBER
    index += 1
  end
end

Benchmark.ips do |x|
  x.report("While Loop")  { fast }
  x.report("Kernel loop") { slow }
  x.compare!
end

Under Ruby 2.4.1 (p111 (2017-03-22 revision 58053) [x64-mingw32]), the difference is striking:

Warming up --------------------------------------
          While Loop     1.000  i/100ms
         Kernel loop     1.000  i/100ms
Calculating -------------------------------------
          While Loop      0.630  (± 0.0%) i/s -      4.000  in   6.350897s
         Kernel loop      0.190  (± 0.0%) i/s -      1.000  in   5.274249s

Comparison:
          While Loop:        0.6 i/s
         Kernel loop:        0.2 i/s - 3.32x  slower

Why is there such a performance difference? And why is the single-purpose loop command worse at its job than the general-purpose while?

(Benchmark copied from here, licensed under CC-BY-SA)