What version of Online Maps are you using?
Online Maps v3.7.1 and 3.7.2 had fixes for some problems with Resources.
If you are using an older version, try updating the asset.
It is quite possible your problem has already been fixed.

I have updated it to v3.7.2, however the problem is still here. There are gif. pictures of the two situations, you can find the one is faster to load tiles.

By the way, can I use the tile caches to load tiles? I don't want to use resources or streamingAssets. I just want to load tiles from a costume path.

I have sent an email to you because I cannot add attachment. Please check, thanks!

Hello.

The larger the map, the more tiles the map needs to download, so the larger the map, the longer it is loaded.
Tiles are very small and, in most cases, load very fast.
In addition, Online Maps uses several tricks to quickly show you the result.
But in some cases, tiles may load slowly, for example, when you have a slow connection, use a VPN or proxy, or you are too far from the server (for example, the server is in USA, and you are in China).
Some problems can be solved by setting up your connection, some problems can be solved only by choosing a different tile provider.

But I just used the resources to load tiles.

The larger the map, the more tiles the map needs to download, so the larger the map, the longer it is loaded.
Tiles are very small and, in most cases, load very fast.
In addition, Online Maps uses several tricks to quickly show you the result.
But in some cases, tiles may load slowly, for example, when you have a slow connection, use a VPN or proxy, or you are too far from the server (for example, the server is in USA, and you are in China).
Some problems can be solved by setting up your connection, some problems can be solved only by choosing a different tile provider.