Well, i think i have found the issue so here’s something we can script to fix if someone else has it.
In each user document, in the service column, there was a 28MB (text version) for a single user. The login sessions (all without dates) were huge. I deleted these for 2 users (and am cleaning up others) and its sooo much faster now.
I presume theres a cleanup routine running to delete old sessions but without dates they will be ignored.
I am not a mongo person but will look at writing a script to delete all sessions where there is no date.
I am also assuming that this has come from an old version and there probably should be a published routine or something to clean some of this up.
I think the logs I had were just pointing at the table causing the issues and locks, not the actual row so the logs weren’t entirely helpful.
Gotta say that I now know far more about mongo db than i thought i needed originally