It has to do with the latest steam update. It was installed yesterday and unfortunately the release has some serious lag issues. Valve is looking into the problem but it may take a few days.
I just installed an old version of the CS .dll on the server. This should fix the lag issue. An unfortunate side effect is the countdown timer on the client displays may not work properly. I don't know how long it will take Valve to fix the lag problem, but at least the server should be playable again.
I just played a while on deadm and had no problems with the timer stuff or lag. I only had a bit of cl_flushentitypacket but that is totally my own error and not from the server.