How to maximize the distance recorded for Portmanteau unlocking

Compared to e.g. a fitness tracker running on the same phone, Wizards Unite only counts a fraction of the distance traveled towards unlocking Portmanteaus.

I’d like to collect best practices on how to maximize that distance.

• Obviously, distances traveled too fast are not counted, but I’m unsure about the actual limit. You’ll get the ‘car warning’ screen at approx. 25km/h, and I assume this limit means that no distance is recorded.
  I have the suspicion that the ‘car warning’ screen also invalidates a certain distance, even if traveling slower again. I tried to completely stop for a few seconds after I received the warning and this seems to work for me, but I still might be mistaken.

• The blue avatar also switches into ‘broom-mode’ when traveling fast. This sometimes happens without the ‘car warning’ screen, so this has either a different limit or some allowed maximum time. I don’t know about its influence on recorded distance.

• The GPS location often loses position for Wizards Unite. When re-locating the player, the missing distance is not counted. Going way slower and stopping once GPS is lost (avatar doesn’t move) helps here.

Has anyone more observations here? Is there an additional factor? Are there technical means to improve GPS to at least the level of other apps on the phone?