Thank you for your reply.

Sadly, attempting to get any support from my ISP has proven to be futile.
I'm sorry for opening this post since it indeed doesn't seem to concern FFXIV directly.

Something interesting to note, is how much my connection suffers whenever it goes through a node from NTT.
For example:

yahoo.co.jp Host: Yahoo Japan Corp, City: Tokyo

160ms

dmm.com Host: DMM LLC, City: Tokyo

157ms

line.me Host: Line Corp, City: Tokyo

216ms on the last node that replies, notice NTT starting from node 5

livedoor.com Host: Line Corp, City: Tokyo

211ms on the last node that replies, notice NTT starting from node 5

Just as bad as clipping the GCD
Thank you again.