Attached screenshot: Me trying the same function on a command block being powered by observer clock. The command worked as expected and I got the right map.
Just realized the "zoom" was at 3. Set it to 1, and it loaded instantly. Maybe the zoom is the problem here and not the styling, or maybe some sort of combo.
May help you figure out what may be happening, or to mark this as a resolved issue or duplicate.
I actually think I have a cool alternative to use for the villager inventories: 1 slot for each item type they carry, and let them keep farming even if that slot is full.
This would satisfy the 2 types of people who want auto-farming to happen:
1) people who want to profit off of villagers who leave their food on the ground
2) people who want a naturally evolving village to not get stuck with a villager full of seeds
In both cases, the players get the benefits they want out of villagers without detriment to the other group. I think it would be a nice solution to these types of tweaks to villager farming.
Tried in 20w17a with zoom of 3, took about 18 seconds to generate the map. zoom of 1 is still near instantaneous.
The zoom being 3 might be an issue I think is not worth solving. I think if zoom of 1 works fine, it is probably a fine issue.