Performance discrepancy between versions 2.7.4 and 4.0.6
Recently decided to try learning Ruby more consistently (before that I only touched it occasionally) - and was much pleased with it in various aspects. At some point I was curious to assess its general performance roughly (as I heard it had to be pretty slow but very long in the past). Then I found my system installed Ruby is somewhat outdated 2.7.4p191 (along with my ubuntu 18.04 on this specific laptop). So I learnt to deploy rbenv and installed 4.0.6 with it.
However I ran into small puzzling situation - I tried two small scripts - roughly doing double nested loop - one with simple arithmetic and another with building and using array (collatz sequence calculations and primes calculation by trial division) - and while for the first of them performance of the said two versions is roughly the same, with the other newer version looks a bit slower.
I'm pretty sure I missed something about versions installed or don't catch something about properly writing Ruby code - so any guidance is quite welcome!
Below come details (with this primes.rb script from my github):
$ rbenv local 4.0.6
$ ruby --version
ruby 4.0.6 (2026-07-14 revision 03b6d3f889) +PRISM [x86_64-linux]
$ time MAXN=1000000 ruby primes.rb
primes[1000000] = 15485863
real0m41,003s
user0m40,985s
sys0m0,018s
$ rbenv local --unset
$ ruby --version
ruby 2.7.4p191 (2021-07-07 revision a21a3b7d23) [x86_64-linux]
$ time MAXN=1000000 ruby primes.rb
primes[1000000] = 15485863
real0m34,969s
user0m34,929s
sys0m0,042s
So you see, it looks like 2.7.4 behaves slightly (about 25%) faster in this case, while I remember (from googling) that there was effort to make Ruby 3.x faster than Ruby 2.x perhaps 3 times. Though I may misinterpret this and perhaps 2.7.4. got the same improvements as 3.x (compared to 2.0 etc)
