Effect of tile size for VAE tiling performance

[stduhpf]

On Vulkan with RX6800 (Windows driver), x axis is the tile size:

Similar results with a RX 5700XT.

According to my tests, the optimal tile size for VAE decoding on Vulkan backend seems to be 26. But I'm really confused about the huge drop of performance that happens for tile sizes between 10 and 25 for no obvious reason. I'm not noticing the same thing on CPU so far (CPU performance seems pretty consistent, with a very slight performance adventage for smaller tile sizes that should not matter in practice)