Sorry for the late respond. Yes, your statement is correct. And I did not see anything suspicious in my log while scanner, fs enabled.
Throughout the time since my last reply, I dug around on the forum as well as github for any similar reported issue.
I even removed all folders and only setup to sync the WhatsApp folder. Problem seemed to be reproduceable.
Found the following post and OP had the problem I am facing. My Samsung S10+ upgraded to Android 11 on Feb 4 which apparently started to give out the issue. I did not plan or want to rollback the OS but I gave it a try to back-out Syncthing.
I managed to find a copy of Syncthing v1.9.0.5 and the scan did not slow down once scanning passes 8000+ files mark. The scanning process for 70K files (~ 40GB) was completed within a few minutes.
What’s the change from v1.9.0.5 and v1.10? I did not have such problem when my phone was still running Android 10 with newer version of Syncthing.