turn off hardware acceleration if its on.

Turning off the acceleration makes no difference. And the speed seems to be very up and down so I'm not sure that reducing the bind list really makes any difference.

Strangely, I just updated the 2926 from firmware 3.9.1.4 to 3.9.4.1 and it then exhibits the same problem. Downgrading back to 3.9.1.4 returns it to consistent full speed again.

Try setting the "Change TTL Value" setting to "Disable", it defaults to enabled but its only needed for a couple of ISPs. Beyond that I've seen reports of virgin media routers not giving good speeds with DrayTek routers and I recall seeing with one issue that using a crossover cable instead of a patch cable helped performance.
It shouldn't be using much CPU for just internet access so something is likely going wrong. If neither of these help, please get in touch with the support team for assistance.

The TTL change doesn't make any difference unfortunately.

If I factory reset the router, and just hook it up with zero changes, a speed test ultimately gives me around 520Mbps. It's a bit all over the place, but that's where it ends up. If I enable the hardware acceleration it gets closer to 550Mbps, but still not very smooth. Using firmware 3.9.1.4 on the 2926 gives me the 550 almost immediately and sits there for the whole speed test (as I mentioned earlier, the later firmware on the 2926 gives poor results).

I then set up my DHCP server and reloaded my IP Bind and it was still fine.

I then set up my DynDNS service and forwarded a port to a server I have for remote camera viewing and the speed dropped to around 500Mbps. I removed those settings and rebooted but I couldn't get back to full speed without a factory reset.

It seems like, for some reason, anything with a recent Draytek firmware just doesn't work well for me.

Could you try this:
Disconnect everything from the router except your main PC which should be connected via CAT 6 (or better) direct to a port on the 2927.
Restart the router and the Hub4.
Restart the PC and let all the usual bootup stuff settle down.

Then for the speedtest download and install the Windows Speedtest.net client.

Run that and see if it's any better.

I went through the testing suggested by MarkPi and it was no different - 550Mbps with the 2926 on firmware 3.9.1.4, 340Mbps with the 2926 on firmware 3.9.4.1 and 335Mbps with the 2927 on firmware 4.2.0.1

After further investigation I've now found that the issue occurs when a Web Content Filter is selected in Firewall -> General Setup -> Default Rule.

I use the Draytek WCF to block certain categories of website and this is what seems to cause the router CPU to be hammered and severely limits the maximum download speed in the speedtest.

The WCF is all set up as per Draytek's guide and, as above, works fine on the older firmware.