Tile downloads get extremely slow after a point

This is on my moto g6 physical device. If this is not reproducible for others, feel free to close, but just wanted to document in case it is:

- Start downloading a large area
- Tile downloads seem to go really well for a period of time
- They seem to start getting slower as the download progresses
- At some point, for me at roughly 10,000-12,000 tiles dowloaded, things get excruciatingly slow to the point of being almost halted -- 1 tile every 5-10 seconds or so.

This is not a regression - for large bounding boxes, I'm pretty sure I've always seen this behaviour. Hopefully it's not something we need to fix right away, just wanted to have it documented in case it comes up / others face the same.

cc @geohacker @mojodna