Large vault on desktop: Loading workspace takes a long time

Hope my observations can help and this is the right place to post it. If not, please move it…

My OS: Win11
My Vault: A Synology Drive Folder on my local hard drive, configured as “sync on demand”, the number of files is about 100k.

This works like a charm: Install Obsidian 1.4.16, configure some plugins if you like, then update to 1.5.12.
Result: Loading vault + cache + workspace in max. 1 min.
Same experience as was reported here: Why does Loading Workspace take so much time? - #9 by gino_m

This doesn’t work: Install Obsidian 1.5.12 from scratch without any predecessors.
Result, even with “naked” Obsidian without plugins, themes etc.: Loading Workspace sometimes successful after 5 min, sometimes after several hours, sometimes never. Not reproduceable / predictable.
Windows Task Manager shows me 4 processes: 3 almost idle, one with relevant CPU load.

It looks like this workspace loading thing came to the world between installers 1.4.16 and 1.5.12, and it resides in the part wich is not altered during an update process.
In my opinion at least the following reasons can probably be excluded. It is: