What kind of router/switch do you have? A couple brands have had issues with autonegotiation for the LAN connection on the C5's ethernet port (I think ASUS and Netgear), and in that case, Support may be able to help you with the fix for that. I think the symptoms are just LAN actions taking a long time, like loading hub admin web pages or LAN integrations like the Hue Bridge app/drivers.
If it's your entire hub, you may have something else going on. For example, are you running any custom code? Some can be poorly written and lead to hub slowdowns. (Only a few have really been known to do that, but it's worth asking if you're not sure or disabling or uninstalling such code to see if it helps.) There are lots of threads on issues likely related to hub slowdowns due to custom code. There are a few scattered mention of the other issue, but if that matches you (and it's not the whole hub) I'd reach out to Support to see if they can help.
