I would recommend checking out the performance tips page here https://help.astraltabletop.com/advanced/performance-tips/.
There are a lot of things you can do to make a large map run pretty well. The biggest one is making sure the background image isn’t all one giant image. Currently, the game only loads assets that are in view, including background map images. If your map background is one giant image, it’ll load the whole thing all at the same time. If it’s split into 5x5 tiles, it’ll only load the tiles that it needs to load at any time. This is true of all assets.
If your image is that large, I suspect the biggest performance improvement you could make would be to export the image and then split it into a bunch of tiles yourself. 264 megapixels is… a lot, so you might need a tool specifically for image tiling. I’m not sure what size is the most beneficial.
Finally, the game map honestly might run fine as it is. It’s a lot of separate assets, but if the map is spread out, it could be fine since, like I said, it only loads assets that are in view at any time.