If you have thousands of photos and movie files that are added into the /photo share, then this will trigger the indexer to create thumbnails and re-encode videos. This process is very CPU intensive and can take hours (sometimes days).
This will happen, regardless of whether or not you choose to not enable indexing on that folder.
which describes how you can stop the process and clear the conversion queue.
synoservicectl --stop synoindexd
When you edit/add/delete files, they are automatically logged in the syno_indexing_queue file. When the indexing service is triggered, it renames it syno_indexing_queue.tmp while it processes it (which can take a while if it hasn’t been triggered in a long time). In the meantime while it’s processing the .tmp file, any subsequent changes will go to a new syno_indexing_queue file.
The bottom line is if you see this .tmp file, theoretically the indexer is already processing it. If you see a syno_indexing_queue file but no syno_indexing_queue.tmp, then the indexer hasn’t been triggered and can be triggered manually with the command above.
You can safely delete all the files in the /var/spool directory, or if you don’t feel comfortable with that, then move the files into another location.