if locus load tiles even when screen is off, then it's for sure a bug. Important is, not to redraw map when it's not needed. If there is even internet connection and downloading tiles when not needed, it's even worst
anyway, I just
1. started locus, started simulation by one NMEA file
2. cleared cache of all online maps, started track record and put locus on background
3. MapQuest_Classic.sqlitedb file (cache of online map), has now just 200kb (one loaded screen)
4. after five minutes, widget of track record display 5km long track, and MapQuest_Classic.sqlitedb is still just 200kb big
so in my case, Locus for sure do not download any tiles. If you'll have time, try this on your own
about improved zooming ... this is not so simple. I was already thinking about some improvements with zoom, to make it faster, so I'll keep this in mind and will try to do something with it ...