Last week I started getting hit with severe lag while dungeoneering with other people. I'd been doing floors with one other person for a while without a problem and then one day I was suddenly freezing every time the other person would pass through a door. Nothing on my end had changed at all, and after lowering my graphics settings and trying a different world, the lag kept on happening and it's made dungeoneering impossible. This is the only area where I lag and nothing I've done, including reinstalling the client, has solved the problem.
My main world is already 0, but I tried a couple others to see if server region or different pings (higher, 0, -) made any difference, and got nowhere. Not only that, but the person I normally do floors with is three time zones away from me and has also started to see a lag, and we tested two different worlds.
I've done a traceroute, changed my game settings, reinstalled the client, emptied and rebuilt my cache twice, and checked on all my software and driver updates. Nothing in that list has shown a sign of being the cause. As far as I've seen, absolutely nothing on my end changed in the month before this specific lag started. My floor buddy mentioned she has also been experiencing the same thing, though her issues only started a few days ago.
When a few people complain about lag issues like these, there is typically a connection issue between their ISPs and the RuneScape servers. You wouldn't be able to do anything about that since the issue is most likely outside of your ISP's and RuneScape's ISP, but rather somewhere in the middle. The only thing you can do while that gets fixed is to use a VPN like Hotspot Shield which would re-route your internet connection giving you a more stable connection.