My friend has set up a Project Ozone 3 server on the same PC as the client. However, when he tries to join his own server, even locally, his ping is at least 1k with 4k+ spikes. I also have a slight delay on that server, but no higher than 500 ms. He doesn't have a VPN, and his network speed is great. Server and client have 7 Gb of RAM allocated each. What else could've caused this problem?
Update: it appears to be a problem of this specific modpack. I don't know what happens under the hood that makes a Skyblock world lag this much, but we tested several other heavy modpacks, and the performance was smooth as butter. So case closed, PO3 itself is the culprit.