It is possible to set netspeed lower than 512k. Look at GET requests that ‘Manage servers’ page send.
pool.ntp.org: the internet cluster of ntp servers will set 1 kbps. I set 1 kbps and it is easy to handle.
upd: set 30 kbps, it gets higher - 50% cpu, about 70k ppm, ~70 mbps. Set 15 kbps, ~45 mbit/s, comfortable load.