I do not recall ever letting the number of items ever surpass 500. When the lag was at the worst, I actually removed every single item from the Alliance Vault but that did not help.
Removing the cargo fighters did not improve the lag.
Deleting all of the stations/mines did improve it to drastically, but we are already noticing it starting to slow down again.
Haven't fiddled around with any of the other suggestions as of yet.