The correction for distance Strava uses if you're on pretty well known roads and routes is pretty accurate. However, you "instantaneous" speed shown on the GPS is most accurate if using a wheel speed sensor with accurate info inserted in for that sensor (rollout, tire size, etc...).

One thing I've seen is that if you're a runner, I have the slowest runs by far on curvy greenway/MUP runs where the curves are so small that the GPS mesh coords are much larger than the curves. Thus, you lose a solid amount of distance that it can't really correct for.

I could assume MTB folks also suffer from this and rollout may work better for distance the curvier your route is.

My most accurate distance/times for running pace are on arrow straight streets and roads with really steady curves to them when they do turn.

Either way, I train on rides to time spent in zones combined with overall TSS. I don't really bother looking at the speed or distance as it is irrelevant sometimes. I only went 18mi yesterday in an hour on a 30/30 vo2 workout but went uphill almost 1900ft. I was chasing X sets of 30/30 and an overall TSS score for the workout. I hit my sets, then free-rode at Z2 and tempo till I hit my TSS score. Then went home.

It doesn't really bother me ever unless doing my super nerd time trial equipment testing. Then I care about the details a bit more.