Cisco Firewall :: Network Is Super Slow After Deny Tcp Log In ASA 5510

I used the ASA 5510 and in these days, facing the problem is internet is very slow. When i check in real-time log viewer debugging, i found the following logs 6|Jun 29 2011|15:47:53|106015|123.123.123.123|416|111.222.111.222|80|Deny TCP (no connection) from 123.123.123.123/416 to 111.222.111.222/80 flags ACK  on interface Inside 4|Jun 29 2011|15:47:53|106023|123.123.123.123|852|111.222.111.222|80|Deny tcp src Inside:123.123.123.123/852 dst Outside: 111.222.111.222/80 by access-group "Internal_access_in" [0x0, 0x0] a lot of log message are come out and I notice that 111.222.111.222 ip is try to attack my network. In that moment, my network is very slow and nearly to be down.  When I block with that ip by access list, network is up again. But after a few moment, attack from other ip, it's so terrible and so tired to block a lot of ip by acl.