Performance issues at high zoom

[richard-uk1]

At the office hours, we discussed the problem with zooming out and an associated drop in framerate. I'm posting this issues so we have a record of the issue to track.

[armansito]

Both zooming in and zooming out result in a drop in frame rate, though these are likely separate issues.

In the zoom out case, the run time of fine increases the most dramatically (I measured 567ms on M1 Pro with paris-30k zoomed out so that the map covers a tiny square). I think this is likely explained by reduced parallelism across workgroups as the entire scene effectively fits within a very small number of tiles and the workgroup size is only 64 for fine.

I see increased runtimes in other stages as well (binning, backdrop, tile_alloc, path_coarse, and coarse all show 25+ ms in the same trace) but the increase is not as dramatic as fine.

Metal capture numbers with paris-30k covering the entire viewport:

Metal capture numbers with paris-30k scaled down to a point: