I am sorry if I didn't understood, but I had an impression that the "old known time issue" is that the router does not change to correct time during the night of DST changing. I had no such experience. My problem is that the router shows two different times - the first as "system time", and the second as "syslog stamping time". I can not see here any connection with DST. Please, correct me if am wrong.
The one hour difference is related to the DST handling. I've experienced the one hour difference as well in one occasion myself, around the time when the DST switch happened, but have been unable to reproduce it again since then. Unsure if it's because part of the firmware uses outdated DST tables while other parts use more up-to-date tables, or just because the timezone gets changed at some point during the boot process, and syslog is unaware of the change until it gets restarted. Not being able to reproduce the issue again makes it hard for me to track it down unfortunately, but there is already an issue on the Github tracker about this.
I have a lot of experience with the same OVPN configuration ran on WRT54GL with Tomato VPN. The WRT54GL is much more weaker as CPU power than the RT-N66U. But I had not any similar observations on WRT54GL. Even your OVPN implementation is based on Keith Moyer's Tomato VPN, so it is unlikely that your OVPN implementation is "guilty". Most probably this issue comes from the original Asus FW. Even without OVPN the Web GUI seems to be slower than Tomato, but with VPN the difference becomes too visible.
Asuswrt's GUI is a bit more resource-intensive than Tomato's, as it has more images, and more complex scripts involved, so it will generate a bit more traffic, which can become more apparent over OpenVPN. I regularly access mine over OpenVPN here, and while I do see a slowdown versus local usage, it's still responsive (I have seen some routers who felt slower even remotely).