This is a limitation of Android with no workaround (see https://forum.syncthing.net/t/how-to-deal-with-abysmal-i-o-performance-on-android-10/18406).
You can still use Syncthing on Android perfectly fine, just not with thousands of files located in a single folder, or otherwise you should be prepared for the slowness.
The common use case is to sync photos and videos, so most people never encounter this problem.