Quick simple benchmark between bsearch_index and index with over sorted list of numbers. Binary search is obvious winner. This is just the proof.

bsearch_index_vs_index_bench.rb

require 'benchmark'

ordered_array = (0..10_000).map { |p| [p,p,p,p,p] }.flatten
random_array = ordered_array.dup.shuffle

Benchmark.bm do |b|
  b.report('index') do 
    random_array.map { |n| ordered_array.index(n) } 
  end

  b.report('bsearch_index') do
    random_array.map { |n| ordered_array.bsearch_index { |x| n <= x } }
  end
end

#
# user           system     total      real
# index          6.281000   0.000000   6.281000 (  6.982050)
# bsearch_index  0.063000   0.000000   0.063000 (  0.058774)
#