Fixed cell height support allows for two things: less time spent calculating the heights of all cells, and the option to virtualise cells belonging to columns that are not visible (in TableView / TreeTableView). From my very early benchmarking, I get the following results:

Using TableView, with 31 columns (7 of which are visible), the FPS count goes from 54 fps to 68 fps when I enable fixed cellheights. That's a decent gain. On the other hand, if there are 5 columns (all of which are visible), the FPS count goes from 67 fps to 69 fps when I enabled fixed cell heights. In other words, there is a only a small gain when all columns are visible.