[Bacardi]

You maybe hit max bandwidth limit, I say maybe because you haven't tell, which sys_ticrate you run and what rates.

Pure guess. I assume now, if you run 100 updates.
Packet size 400 Bytes X 100 updates = 40 000 B/sec per player
40 KB/s * 15 players = 600 KB/s
600 KB/s * 8 = 4800 Kbps
4.8 Mbps

So, you can try limit players max rate:
sv_maxrate 40000
sv_minrate 30000
- this however cause choke to players when there is much action on server.

Another way is drop your updaterate:
66 updates X packet size 400 Bytes = 26400 B/sec per player
26.4 KB/s * 15 players = 396 KB/s
396 KB/s * 8 = 3168 Kbps
3.168 Mbps

So
sv_maxrate 0
sv_minrate 30000
sv_maxupdaterate 66
sv_minupdaterate 30

If you change rates, you should change map so players reconnect to server and server capped rates start working.

CPU could be another reason, why lag start but... try easier thing first.