if your subs can be broadly categorised as disk-intensive, memory-intensive, cpu-intensive you can see which of these 3 groups varies substantially against the others (standard deviation is an indication of variation). For example that YAML* sub being invariant in all three runs while others with similar run-times were not. Perhaps perlperf can help you.

Also, are you 100% sure that your program is deterministic? Are there no random choices? For example, even iterating over the keys of a hash is non-deterministic. What if the algorithm benefits if the longer keys are processed first? Also, regex are also working internally with non-deterministic algorithms (that's my understanding on when a regex consists of a "choice") and therefore may show variability in termination (see https://stackoverflow.com/questions/36420517/is-it-faster-to-use-alternation-than-subsequent-replacements-in-regular-expressi).

If you want to exclude the possibility of non-deterministic behaviour and other program-specific factors, start by doing some benchmarks yourself. See Benchmark on how to profile subs yourself easily.

People from my social circle constantly harass me with their windows-10 too-slow problems. Sometimes I find that they have background updates sucking up resources (sometimes it's a cryptic generic windows process name).

bw, bliako

Thanks for the questions!

The loop that is taking all the time is mostly cpu intensive with a decent memory footprint (but no where near my laptop limits)
Its runs a large loop of loops performing string compares, index functions and regex on each element.

I am reasonably sure that the test code is deterministic.
There is some usage of the keys function to order the way we go through the loop,
but on hashes with very few keys and the order of the keys really don't affect anything.

I think my data-point about the effect of opening gmail on a freshely rebooted laptop is the most revealing item.
I've been playing with my power profile, but It doesn't seem to be doing any good... the fan goes up and down throughout the longer tests.

In windows it's all a big experiment ... turn wifi off, updates off, email, browsers, a neverending catch-me-if-you-can with the phone-home fairy.

Anyway, I have no idea on the internals of Perl regexes. It looks like they are the only non-deterministic element in your code, with the hash key search. And, in theory at least, they can contribute that variability in time you observed. Perhaps. It's worth checking min, max and stdev runtime of each of your regexes.