I'd like to point out that there is a 10ms ping overhead between MabiPro's public IP address and our actual game server, so do take that into account when measuring ping from command prompt.
Local 0ms ping test server with Nagle's algorithm enabled
http://mabinogi.ir/img/vid/mabinogi_2017_03_07_001.webm
Local 0ms ping test server with Nagle's algorithm disabled (Theoretical fastest speed)
http://mabinogi.ir/img/vid/mabinogi_2017_03_07_002.webm
Remote 40ms ping production server with Nagle's algorithm disabled
http://mabinogi.ir/img/vid/mabinogi_2017_03_07_005.webm
There is a minuscule difference between production and test server as you can see by my videos and Nagle's Algorithm is a serious ping detriment so make sure that is turned off.
I don't see anything wrong with your 1500 MTU test, it is damn near close to the theoretical fastest speed.
The reason your skill actions are delayed is because of Mabinogi's built in packet queue.
Your 128 MTU test has inconsistencies that are caused by your local computer not dispatching packets as a whole. Basically, your MTU is too low. It's splitting up your packets into smaller segments which causes a significant delay during full transmission. MTU is not a packet queue size, lowering it does not cause packets to dispatch quicker.
There could be minor networking inconsistencies between you and the server, but that is completely out of our control and you should contact your ISP about that; I don't see any inconsistencies however.
Here's the point: As long as your client is sending packets properly, packets are arriving in a timely manner, and you don't hit Mabinogi's built in packet queue that delays packets intentionally if you send too much (like autoclicking), you will not experience lag.