So I have a very unusual problem. I have set up a minecraft server for the family to play on. We have four computers. Three of them can connect fine to the server. The fourth one however can not, when it connects it lags out completely, even if it is the only computer using our internet connection (100Mbps Down, 25Mbps Up). It is actually the EXACT same model/spec as one of the other computers, and this other machine does not exhibit the same problem.
This problem computer can run minecraft fine in single player; it can also run minecraft fine when connecting to another machine on our network where I have started a single player game and done 'Open to LAN'. The problem computer can also connect to for example World of Warcraft and have no lag problems.
Server is running vanilla minecraft 1.8.x (I've tried a few different versions, all gave the same problem).
All computers running minecraft 1.8.7 with the appropriate optifine.
I think I've provided all relevant information, if there is anything else I can add please let me know.
Best Answer
So we have a solution (all credit to my better half for figuring this one out). The solution was to go in to Configure Java -> General -> Network Settings and change it from Proxy using the browser settings to Direct Connect. Not sure why it was set that way to begin with, but that sorted it.