I wonder how much more % it takes for this addon to load a page
Possibly you would need some benchmarking
for this, I actually didn't make tests.
But it's purely just about more maths. Considering the current date conversion function already implements some re-calculations
and maths for any date, I would expect the calculations increase is very small, specially for powerful processors.
Maybe it would be even faster if it was done directly as a function
It's already done as a function.
replacing the current default one
You could not replace the default one. "Time-ago" function will stop at some range, for some older dates. It only substitutes seconds/minutes/hours and a couple of days for the most recent dates. Older dates are displayed in default format.
Congratulations on the first 1.5 decade, at least 2 more lapses to go ;).
Thanks, I didn't even notice how these years passed actually...