Guy has typical problem which might have many sources, but your replies dissapointed me.
I afraid i can't answer what's wrong but here are some facts:
Have u tryed to restart the server ?...
Yes this can help.
it can he just need open some ports who allow upload and download faster.
They have nothing to do with "lags"
If i'm right he means lags as high ping / spiky
Dont adjust the tickrate to as much as your upload speed can't handle!
Tickrate has almost nothing to do with bandwidth ( upload speed)
Though tickrate 100 takes nearly twice more CPU than 66.
It's well known that 33, 66 are standart hl2 engine tickrates while 100 is overloading and can easely cause unstability (spikes/crashes) especially in combination with WCS.
A WCS cannot run properly as a dedicated server (i mean hosted at home)...
Dreams.
Any computer can be in a role of "server" and when u said "can't be hosted at home", should he move his PC outdoors in order to host?

P.S.
I would recommend:
checking CPU usage (if it overloads here is the reason)
If you run SRCDS on same PC where u play or run many other programs i will recommand giving SRCDS process higher priority (can be done in windows taskmanager window)
Turn on network graph and make sure there are no package loss
And u better explain what is "lag" in your situation, this will help debugging.