Low frame rates when using custom 2d tilemap

When not using the tilemap or just having the "select area of grid" tool with the tile palette open, my unity project runs completely normally. But whenever i simply click the paint tool or the fill tool, the frames per second of my unity project plummets down to 1-2 fps. It makes painting the map a very very huge chore as it takes me way too much time to do something that can simply be done in a minute or 2. When i use free tilemaps off the unity store i do not seem to have this problem at all. This problem only arises when im using my own tilemap, where each tile only has a 128 x 128 pxl size so i doubt their file size is overloading my GPU.