To start, I'm not really a professional when it comes to networking, but I know the basics to lessen the lag in game.
When it comes to Dungeons, the host controls the lag of the monsters. If you're a host, please be sure that your modem can support the game's function. If it can't, you can decrease the quality of the game by pressing F1 a few times (sometimes, the game will auto adjust the quality of the game depending how bad you're lagging). You can also adjust the quality by going to the Options Menu at the top right of the screen. If you would like to increase the quality, you can press F2.
If you're using Windows, press Ctrl + Alt + Delete and look under the Progress Tab. If you see that there's a program that's taking a lot of memory, you can try closing it (warning, don't close any program that helps function windows). If you're running any downloads or torrents, close them as well. Most messengers such as AIM won't take much memory, but depending on what you installed for Windows Messenger (MSN) or Yahoo Messenger, there could be additional applications taking up more memory. If so, close them before playing the game.
If you're running any browsers like Internet Explorer or FireFox, close them as well. If you need it open for notes and such, I suggest saving them for offline use, then opening the saved file. Otherwise, if the notes are in text, save them on notepad/wordpad.
If you have a wireless router, I suggest you read up on how to keep your wireless router on private. There are times when people nearby may be using your wireless router, causing lag when you play the game. Add a lock/password to prevent any strangers from using your modem.
Stolen from Nexon Forums - Fix Your Lag or Just Improve Latency!
which works for Grand Chase (requires Service Pack 2):
Originally Posted by Darkmgl
This is not a joke. I have been suffering from major lag for a long time, and I have FOUND the problem.
The issue is something known as the "Nagle Algorithm". Anyways I'll keep it simple, this algorithm was created years ago as a way of allowing a steady stream of data to use low bandwidth. It was designed for older dial up modems to prevent online games and other things sending large numbers of small packets from filling up your entire bandwidth sending "ACK Packets". What this does is automatically delay your computer from responding to a single packet by 200ms which allows your computer to respond to multiple packets at once using less bandwidth. What this essentially does, is add up to 200ms to your lag. If you want to know more, do some googling; I don't want to bore you.
Anyways, if you are on a modern broadband connection, you can DRASTICALLY improve your Mabinogi lag by disabling this algorithm. I am unsure of how to do this in Windows Vista, but in Windows XP it is a simple registry edit.
1. Run Regedit
2. Browse to HKEY_LOCAL_MACHINE \SYSTEM\CurrentControlSet\Services\Tcpip\Parameter s\Interfaces\
3. It gets tricky here, there be a bunch of Folders with letters and numbers under this Key, you need to find the one that is your network adapter. Browse them one at a time and look for the ones with similar settings to your network, such as if you have an IP address or a Gateway manually set. If you are unsure, you could always just try adding the line to all of them.
4. Add a DWORD named TcpAckFrequency This is Case Sensitive. If it is already there, skip to step 5.
5. Set the Value of the DWORD to 1
6. Reboot your PC
After this, the Nagle Algorithm will be disabled.
If you wish to test your lag, the best way to do it is by casting the Healing spell. Try casting all 5 charges of your healing spell before this fix, then after. I guarantee you'll cast them twice as fast after this fix if you have had lag issues or are living on the east coast.
For more information read: http://smallvoid.com/article/winnt-nagle-algorithm.html
I was ready to quit this game due to lag issues until I found this. Now all of my problems have completely gone away. If this improves your game performance, please share!
Ignore the test.