Small WISP with customers connecting to CCR edge router (gateway) using PPPoE. Before the CCR I used RB1100AH.
In the PPP Profiles I have used the internal address of the gateway as DNS address, and this has worked well. Static DNS settings in “IP DNS” in the CCR and “Allow remote requests” checked.
This way gateway and DNS address handed out to customers are identical.
However, lately I have noticed a considerable delay when doing DNS lookups via the DNS cache in the CCR. When navigating to a webpage, it takes many seconds to start load the page and sometimes the browser stops with an error message.
Tried to set static DNS on computer and then everything is blazing fast so I started to suspect the DNS cache in the CCR as the guilty part.
When pinging hostnames from the gateway it lags, but when lookup has been done and pinging IP addresses, ping times are normal.
Other operations not involving DNS does also look normal.
Tried to flush cache and then upgrade from ROS 6.7 to 6.9 but still no luck.
Finally I specified static external DNS addresses in the PPP profiles, and threw users out, then everything is ok. Web pages load instantly.
Any idea what might be wrong? 
As mentioned I have used DNS cache since day one, so why did it start to mess things up now?