Have you tried using the profiler?
https://wiki.gdevelop.io/gdevelop5/interface/debugger/profile-your-game/
It might be equal or better to use 1 tilemap over 1,000 objects except I don’t know if this would be an issue but objects offscreen use fewer resources. If part of a tilemap is always on screen then it might prevent this. Honestly, I don’t know if that would be an issue.
I remember reading about sprites vs tiled sprites. IDK if it applies here as well.
https://forum.gdevelop.io/t/tiled-sprites-vs-normal-sprite/56153/7?u=keith_1357
Unlike tiled sprites, tilemaps probably have builtin functions to compensate for this.
The best suggestion would be to test both methods unless an expert has more knowledge of the internal workings.