Hello, thank you so much for reporting the issue! I tested a project with 256 frames, and there is indeed a significant performance decrease in v1.0 compared to v0.11 when changing any kind of setting in the export dialog (which includes the amount of rows/columns in a spritesheet). This is due to the new layer blending system that supports blending modes, which did not exist in v0.11. However, it is possible to make this faster by simply caching the blended frames once when the export dialog appears, instead of re-blending them every time something changes. I am currently working on this and it should be available in the next version, so you shouldn't experience any slowness when changing the number of rows/columns.
As for the 6 rows turning to 12 issue, I cannot seem to replicate it. If you have any exact reproduction steps, it would be very helpful!