This issue originated in firmware 3.7.1 and 3.7.2. Firmware 3.6 does not have this issue.

If you use tagged VLANs in the 3910 and native IPV6, then DNS resolution becomes slow/unresponsive ONLY on the tagged VLANs, but only where the client is using IPV6 as the preferred protocol. If you disable IPV6 on the client the DNS response is normal. Untagged traffic to the WAN does not have this issue. If you use only IPV4 or your clients default to IPV4 then you will not see this issue.

Should you experience this bug, then a downgrade to firmware 3.6 will resolve your issues. It has taken weeks of testing to understand this bug. 3.7.2 hints that DrayTek has seen this issue, but sadly not yet resolved it.

I think that there weren't enough reports of this, so DrayTek haven't been aware of the problem?

Thank you for looking into the root cause of this problem and reporting it to support.
I can see there was another report of it back in October, but it didn't have a useful resolution. In that instance they did what you have and downgraded the firmware. It looks like that didn't get reported to the firmware team for a resolution. Sadly that means future firmwares (i.e. 3.9.7.2) also have the problem.

The Vigor 3910 3.9.7.2 firmware is currently in testing to be released in the UK. So the specific conditions you've defined as the problem will be tested out, to reproduce the issue and hopefully it can be resolved quickly.