I've explored this pretty extensively and it's currently just the way it is. It happens from first sync so the rescan interval makes no difference. Your collection probably requires 300-400MB of RAM to sync so it should be on the cusp of failing on a raspberry pi.
Here's the bug report that was closed without any fix:
A sync tool that uses on the order 2MB of RAM per GB of files or 4kB per file isn't really usable. A cheap NAS these days can easily have 6TB of RAID6 storage but it won't have 12GB of RAM. Clearly something in syncthing is keeping way too much stuff pinned in memory and the developers so far haven't focused on that. There's no apparent need for this memory usage either as there's already an on-disk database with the state of the sync folder.
Here's the bug report that was closed without any fix:
https://github.com/syncthing/syncthing/issues/468
And user reports of this show up pretty often:
https://github.com/syncthing/syncthing/issues/1841
https://forum.syncthing.net/t/arm-linux-cpu-usage-99-for-man...
https://github.com/syncthing/syncthing/issues/1841
A sync tool that uses on the order 2MB of RAM per GB of files or 4kB per file isn't really usable. A cheap NAS these days can easily have 6TB of RAID6 storage but it won't have 12GB of RAM. Clearly something in syncthing is keeping way too much stuff pinned in memory and the developers so far haven't focused on that. There's no apparent need for this memory usage either as there's already an on-disk database with the state of the sync folder.