The Boss has 10 HP per phase.

As long as you collect the first 9 tiles within about 1 second of the moment they spawn, you're doing perfectly. A tile spawning under your feet will be no faster than running to collect it from the corner in 0.9 seconds.

You can only really save time on Tile10---the tile that kills the Boss---because his damage/death is immediate, unlike the tile spawns which are fixed relative to the clock. Tile 10 should be collected as quickly as possible.

The perfect run would require Tile10 spawning under your feet in every phase. The time saved by this RNG would be pretty small (maybe 2 seconds per run), but if you're trying to save every second possible, it is essential.

If you want to see this concept demonstrated, you can check out my 3:21 Hard Mode Glitchless run. 3/4 of the final tiles spawn right under my feet, which is how I was able to get 3:21.

(You can also see that I almost ragequit and closed the window after the 10th Tile spawned relatively far away in Phase 2.)