Is _ as an argument name optimized or just a convention?

benchmark.rb

def does_care(*a)
  "hi"
end

def cares_a_little(*_a)
  "hi"
end

def does_not_care(*_)
  "hi"
end

def really_doesnt_care(*)
  "hi"
end

require "benchmark/ips"

Benchmark.ips do |x|
  x.report("*a")  { does_care(1,2,3) }
  x.report("*_a") { cares_a_little(1,2,3) }
  x.report("*_")  { does_not_care(1,2,3) }
  x.report("*")   { really_doesnt_care(1,2,3) }

  x.compare!
end

result.txt

Calculating -------------------------------------
                  *a    52.377k i/100ms
                 *_a    52.644k i/100ms
                  *_    52.383k i/100ms
                   *    52.646k i/100ms
-------------------------------------------------
                  *a      1.785M (± 7.2%) i/s -      8.904M
                 *_a      1.777M (±11.7%) i/s -      8.686M
                  *_      1.810M (± 8.0%) i/s -      8.957M
                   *      1.811M (± 6.9%) i/s -      9.002M

Comparison:
                   *:  1811206.5 i/s
                  *_:  1809693.2 i/s - 1.00x slower
                  *a:  1784987.0 i/s - 1.01x slower
                 *_a:  1777387.3 i/s - 1.02x slower