File Access slows down every day for around 20 minutes at around 15h15

Users that access files on the server have shown me that access to their files slows down to a frustratingly slow pace every day at around 15h15.
How do I isolate this issue, what can I do to see what's causing the issue?

I run a UDM Pro SE and a full Unifi network and ping times from the user to the server a 1ms. Internet speed is 500/200. All switches are connected over fiber, not an ethernet cable.

Thanks for reading. I am a novice and not a certified network engineer so please, any advice would be appreciated.