I've just updated a 3900 to a 3910 and I can access the managment page remotely as I could with the 3900 but its terribly laggy and quite often times out. If I VPN and access it via its internal IP address, it works fine. Its on the latest firmware

Any ideas what this could be?

Nope but then the 3910 is very buggy. I haven't had the issue. You might nas well post in warning 3910 firmware issues.