Minecraft, why you only work for me when server is empty?
I keep meaning to respond to you.
There are a few things you can try to find out where the issue is.
First thing to do is make sure your network card driver is the latest one.
sometime the card after updating have some new features like jumbo frames
if you have the option set it to the 9K this make packets work faster.
Second make sure your router has the latest firmware. This can bring the security protocols up to the most current version i have seen old firmware cripple networks many times.
Third. Do a ping test. ping
www.bbc.co.uk
use the following command in command prompt.
ping
www.bbc.co.uk /t
that is a continues ping and wont stop until you close the window.
keep an eye on the speed. see if it jumps up while playing or browsing web site. a small jump is fine but a large jump and staying up there is not.
If you are on wireless find out what channel your Access point is using
normally in Ireland most of them are set to auto or channel 11.
if on channel 11 you may be getting interference from other wireless AP's in the area. if you can change the channel to 1,6 or 13 if you have channel 13 that is. the first 2 are non over lapping channels and should reduce any interference.
If you have a dect phone. (a wireless phone) try unplugging it for a while. Those phone use near to the same frequency as wireless devices and cause issues.
There are loads of other thing that can cause this too but that should be enough to get you going.