This may help some people, but at least some of the lag problems are definitely on the server side. Channel 5 crashed a few hours ago while I was on it, ever since then I haven't been able to run + grab ladders. Before that channel crash I was doing so with no problems. This isn't the first time I've seen this happen either, I suspect the channel crashes are either causing serious server instability or it's some kind of serious server instability that causes the channel crashes. Either way it generally doesn't improve until they do the next update, which is probably cause the server gets rebooted then. Do note, I'm not saying don't try this, just pointing out that some of the issues aren't fixable by us, we have to hope OGP will fix the server too.
Another thing you can check for to help improve your lag: make sure nothing is doing any uploading while you're playing. Even if it's not using much bandwidth it can still cause horrible lag, the La Tale client is apparently extremely
sensitive to upload latency. I discovered this because I use BackBlaze for backups and while it never causes issues in other games, with La Tale it does. I have to make sure I disable it on all my PCs while playing. Also you may experience problems with upload latency during peak traffic times if you're on cable. The way cable internet works there can be problems with upbound network capacity from your neighborhood. I see problems every day starting around 5pm once people get home from work. Fortunately I can generally still run + grab ladders but I can definitely tell when more people start using the Internet.