We have 3 AD DC with DNS running on Server 2008 R2. Users are on dynamic DHCP. Users started to experience a few weeks ago that browsing on the internet is slower.
If the users just have the in house AD DC as their only DNS servers. Name resolution works fine for same subnet and the remote subnet, but suffers on the web browsing.
If we add the ISP DNS server to the DHCP scope. Name resolution works fine for same subnet but suffers for name resolving on the remote subnet. Browsing the internet no issues.
Servers are all current with patches.
I have run dcdiag and all came out clean and passed all DNS tests.