As one of the poor users of this system...
I've had the joy of having to use CJSM since getting a promotion to a new role last year, but my colleagues who've been on the team several years tell me it's always been slow.
This week, however, has seen performance drop off a cliff. Whereas before it was slow but would at least do what you ask it to eventually, this week, especially during peak office hours, it's essentially been unusable, with frequent connection reset errors, unexplained logouts etc.
The only reason we use this mess (max of 10MB attachments on an email, have to upload attachment files one at a time, file downloading bug that sometimes results in an error in Chrome, stupidly small mailbox size limit meaning we can't leave emails with attachment sin the box too long or else we can't receive any more incoming emails) is because we have a number of colleagues who work essentially part time/zero hours who aren't classed as permanent employees and so IT won't set them up with corporate email accounts so they could use something like Outlook Web Access.