We have 6 sites all using 2820N.

All are connected to each other by IPSEC VPN.

One site has good internet connection and can ping websites with good response times, however pinging other sites over VPN is awful 2000+ms.

All other sites can ping each other with good response times, but the all get slow responses from the site in question.

Any ideas why VPN connections would appear to be "flooded/throttled" when the overall bandwidth is ok ?

edit : I disabled QOS and the response times are now 500ms. Much better but still far slower than it should be. Possible that there is a backlog of traffic but that doesnt explain why the internet in general is and was fine (there were no classes defined in QOS)

Are you using Medium (AH) security or High (ESP security? I recommend using High ESP since the routers do actually hardware accelerate that. The 3.3.7 firmware should be best to use with that.