I'm getting 19-21ms response for all the SURBL lists.

 

They do throttle you if you exceed 250,000 messages per day, but if you are using your own private DNS Servers it more likely it sounds like a latency issue somewhere between you and them (a tracert should tell if there is a routing issue or node timeout somewhere). Considering their servers are in the Netherlands, and Italy periodically slow response times wouldn't be too unusual if you are stateside.