When you only want to substitute one thing use `sub` instead of `gsub`.

gistfile1.txt

Rehearsal ----------------------------------------
gsub   0.013451   0.000356   0.013807 (  0.013817)
sub    0.005154   0.000086   0.005240 (  0.005302)
------------------------------- total: 0.019047sec

           user     system      total        real
gsub   0.011792   0.000351   0.012143 (  0.012186)
sub    0.003583   0.000001   0.003584 (  0.003583)

gsub_sub_bench.rb

# frozen_string_literal: true

require 'benchmark'

HOST_WITH_PORT = 'example.com:80'
HOW_MANY = 10_000
REGEXP = /:\d+\z/

Benchmark.bmbm do |x|
  x.report('gsub') {
    HOW_MANY.times do
      HOST_WITH_PORT.gsub(REGEXP, '')
    end
  }

  x.report('sub') {
    HOW_MANY.times do
      HOST_WITH_PORT.sub(REGEXP, '')
    end
  }
end