There is a general slowness issue with details panel in tile set editor; even though tiles array is hidden except for one element, details panel checks every element every frame. However, given your numbers we’re only talking say 16x19 = 304 tiles or thereabouts, so I wouldn’t expect it to be too bad. Can you hit ` key and type stat slate, and take a screenshot of stats? You can also try closing details panel to see if that was issue (it can be brought back using Window menu).