gh-784: Improve reg test reliability

[connoraird]

Description

Use mean instead of min with a more lenient threshold and use different timer to exclude IO and sleep

Brought over from glass-benchmarks repo PR glass-dev/glass-benchmarks#20

Closes: #784

Checks

• Is your code passing linting?
• Is your code passing tests?
• Have you added additional tests (if required)?
• Have you modified/extended the documentation (if required)?
• Have you added a one-liner changelog entry above (if required)?

[paddyroddy]

10% seems better and hopefully this will prove stable