Problem Solved?!
I think I had the same problem...
I wasn't able to walk without lagging, open doors working well, too, but when I was clicking on chests, or BC/TE machines to open their interfaces it happens between a minute and two minutes later than I've clicked on them.
When I was clicking on a chest for example, my buddy everytime told me, that it is already open, but I can see it in about a minute later.
We've checked the RAM assigned to the server and it's used by 60%. Even the processor is just "chilling around".
My PC uses 10% of it's CPU and 5% of it's RAM.
We've tried to start a Tekkit Lite server. Tekkit Lite was running without problems until the Launcher update yesterday, but now it's as laggy as the actual Tekkit pack.
Then I proofed my options again an saw "Servertextures: ON" in my Video Settings and put it off.
After that it was working!
I'll test it for a while and if it doesn't help I'll tell you =)
Greetings,
Alzheimer1992
EDIT:
Worked for a few minutes, then started lagging again...
My buddy told me it could be the internet connection, but it has worked for months.
I hope someone could help with that^^