Dashboard running slow for 1 sub site

I have a client who runs a couple of sites on my multisite install.
One of there sites admin areas is running really slow, while the others are fine. When I check it my end all seems fine. Though they say they have the same issue on 3 pcs in 2 locations.

I read it could be the db_version didn’t update properly so I’ve checked that in the wp_options table and that is correct.

Also did a network update.

Any thoughts?