dns-server performance

Hi,
i have mikrotik rb1100AHx2 as dns-server. On table it work fine for me.
But on live network, when rate of requests more 500 queries per second, board answering very slow.
its realy use RB as medium-load dns-server (about ~5000 queries per second)?
On the same place unix-box work perfect.
my settings:

/system resource print 
             uptime: 4h13m12s
            version: 5.22
        free-memory: 1506056KiB
       total-memory: 1555212KiB
                cpu: e500v2
          cpu-count: 2
      cpu-frequency: 1066MHz
           cpu-load: 3%
     free-hdd-space: 89984KiB
    total-hdd-space: 110212KiB
  architecture-name: powerpc
         board-name: RB1100AHx2
           platform: MikroTik



/ip dns print 
                servers: 8.8.8.8,8.8.4.4
        dynamic-servers: 
  allow-remote-requests: yes
    max-udp-packet-size: 4090
             cache-size: 512000KiB
          cache-max-ttl: 1d
             cache-used: 3178KiB