Map tiles are downloaded from free, community-run OpenStreetMap services as you pan and zoom. When they arrive slowly, the service is busy. There is no faster path the app can take on your behalf.
It only happens once per area. TrailRunner Mini caches every tile it loads. An area you have already looked at draws instantly next time, and works with no connection at all.
Choose Map › Preload Visible Region. It downloads the tiles that are missing in the visible area and leaves the ones you already have alone.
Tip: do this before heading somewhere without signal. Load the area at home, and the map is there on the trail.
A cached tile stays as it was the day you loaded it, and OpenStreetMap changes constantly. Choose Map › Refresh Region to revalidate just the tiles on screen and fold in what changed. Your cache is kept. Only the stale tiles are refetched.
Map › Clear Map Cache purges the entire downloaded tile store and reports the space it freed. Nothing is lost permanently, because tiles re-download as you next view each area. You do give up the offline coverage you had built up, so reach for Refresh Region first.