You're probably experiencing lag in big cities because of the amount of players running around there doing things that might stress your performance, duels for instance. The megaserver is split up into 'shards' (thus so are the players) and things can get laggy when the shard fills up. When a shard gets too crowded, the system automatically creates a new one and moves new players there. This is something you normally don't see, but that you can take advantage of when lag is bothering you.

You see, it's possible to switch shards by teleporting to other users (by right-clicking their name in your friends/group/guild overview). This moves you to the shard they're currently in. Now it's entirely possible this person is in the same shard as you are or in one just a busy as yours, and there's no way of knowing. But if lag is really bothering me, I give it a try since it only takes 1 min. and the teleport is free - as opposed to a wayshrine. Logging out and back in should in theory place you in the newest (emptiest?) shard, but this obviously takes longer.

It's not really a 'fix', but it might help you understand where the lag is coming from and how to work around it.