[ratelimits] Double CPU usage with RRL

[William Taylor]

I'm seeing approx double the amount of CPU usage when RRL is enabled.
CPU usage is normally around 50%-60% for named but when RRL is enabled
it jumps to 110%-130%. Just wondering if this is expected and I just need
to throw more CPU at it. This is on a recursive name server btw.

Below is the config I'm using for RRL.

       rate-limit {
               responses-per-second 60;
               nxdomains-per-second 0;
               ipv4-prefix-length 32;
               log-only yes;
               min-table-size 200000;
               max-table-size 300000;
       };