Media Library / File Upload Issues on Multisite

I’ve got two sites on this multi-site – bulkingtoncongregational.org and arcchemel.org.uk. The Bulkington site seems to be working fine but the arcchemel.org.uk site is having issues with the media library and file uploads. It’s taking several minutes for the media library to load at times and if I try and upload files it gets stuck. Sometimes a refresh will show the file is there but not always.

Any ideas where I can start to troubleshoot this as it’s on the same multi-site set up as a site that’s working fine?