Hmm. We notice frame rate drop and lag on the oldest machine (only).

On your exit question, I assume you're referring to what most games offer which is a choice between waiting while your character logs out or exiting to the desktop immediately but with your character remaining in the game while the exit counts down as per the first option. The advantage of the first option is that if you get attacked while counting down you can react, plus there's sometimes a suspicion that an instant exit might not always save your character's data correctly, so it's usually better just to take the first option and wait for the logout to be completed normally.

If changing direction makes your game stutter, your graphic settings are WAY too high. As this is normal basic gameplay. This also translates to the same with the dolmens, although that is more graphically heavy. Lower your graphic settings until the problem is gone. Start with settings like shadows, view distance, particle effects, and other small things you barely notice.

When setting your graphic settings, test them during graphically high stress moments(dungeons/PvP), to make sure they are set correctly. If you only test them in town or overland, and you never tested them during high stress, there is the risk you could have placed them too high. This can also cause your skills to look delayed, when they are actually not. They can only appear to take longer to fire, as your GPU catches up.
(Note: Testing your graphics capabilities during PvP is best, as that is most often the most graphically heavy situation you can be in. But I always test mine during dungeons, as that is the second best option. When you get it running great, make sure to lower your graphics settings just some tiny bit more, to make sure that if you do ever get into an even heavier graphical situation, you can still run the game without issues.)

Not sure how to explain the lag, maybe more people in your area or LAN are using the internet a lot. Meaning yours will be somewhat slower.

When logging out you are presented with two options. Normally waiting and logging out means the server will log out your character cleanly and saves the data without problems. The second option to instantly log out, is there for when the gameserver or your client has a problem. For example if the server crashes, you cannot log out, and you will have to force a log out this way. Or when your internetconnection is interrupted, and you cannot tell the server to log you out, this option would still allow you to close the game.
Do not worry, even though the waiting log out is more clean and makes sure the server knows you are gone. The second option will eventually notice you are not connected, and will log you out. Or it will notice you are AFK, and will log you out after 15 or so minutes. Unless you re-login, you will not notice this though. But only use the second option if you really have no other choice.

PS: Have been playing quite a lot on PC EU, no framerate drops or input lag.
PPS: Make sure no background programs are running, or that noone else is using your LAN. (If other people are using your LAN, limit their bandwidth, or set a minimum bandwidth for your own device.)