Server lag since update

Hello,
I have a problem with my gmod server since I updated to Debian 7 (maybe a reason for the lag is the gmod update, too). When there are 15 players on the server, the CPU is at 100% and the pings will get higher and higher. Before I have updated there had not been any laggs until 20 players.
Can you help me, please?

Here a the system data:
OS: Debian7
RAM: 16GB ECC RAM
CPU: 2x AMD Opteron CPU | 8 CPU Cores – 8x 2,3 GHz

Maksimo007