Tyres?
I have a 2 year old ADV9.0 on 38mm Gravel Kings and it zips along lovely
What do you normally ride?
Obvious things
- Handbrake off 😜
- Sticky BB
- Really heavy/wooden wheels/tyres Vs what you're used to
- Position not dialled in yet. This is where my money is if you're not normally a drop bar rider. The right position on a drop bar bike always seems more critical to me than on the MTB for developing decent power. If you're a roadie type you'll know that already of course and apologies if teaching you to suck eggs.
- Tyres pumped up too hard
Tyres/tyre pressure the obvious thing to check.
Try swapping wheels if you can borrow some that fit. Knackered hubs or bearings might spin freely unloaded, but drag when loaded.
Are the brakes dragging when the bike is loaded? When you sit on the bike, the frame and wheels flex. That can cause brakes that don't drag unloaded to drag when you sit on the bike.
Pedalling in treacle
That’s nothing. A mate of mine said he’d been waterskiing in grease.
I'd start with the tyres, they can make a huge difference.
Are they tacky, knobbly off road tyres that you're riding on tarmac?
Cheaper & heavier wired bead tyres can be sapping too.
Once you have experienced low rolling resistance tyre setups, previously reasonable tyre setups can feel like riding in treacle.
While GP4Seasons serve a purpose for winter riding, they felt so slow compared to GP4000SIIs and GP5000s on my road bike.
Recently I swapped from rear wheel 40mm Marathon Supreme with butyl tube to 45mm Caracal Race with latex tube (Vittoria MTB 1.7-2.3") and my gravel ebike feels zippy on tarmac under 25kph motor cutoff and when I'm powering it alone at higher speeds.
I've recently tried a 50mm Schwalbe Century and butyl tube up front on my gravel ebike instead of the 35mm Marathon Supreme and while it made the ride more comfy, the bike felt dog slow.
I suspect tyres. A few weeks ago I dug out my old Cotic Soul to use as a shopping bike (i.e. a bike I can leave locked up without fear of it getting nicked. It had got 2.35 High Rollers and my first couple of rides were surprisingly hard. But I think that was down to me trying to go at my "normal" pace. It's that last 1mph that's the trouble. Once I twigged that and road at the bike's natural pace it was a lot easier. But then I changed to some thinner ones that I found in the garage (blimey it's hard to get 26" all-round tyres) and a ride to town and back was actually pleasant. I made sure they were well pumped up though.
Please do what @alan1977 suggested first and check it’s not a stiff bottom bracket bearing which may be worn or the bearing free play adjusted incorrectly. Check also that the pedals spin easily.
A poorly lubricated chain, faulty jockey wheels can also cause unnecessary friction under load.
I would then check that the axles are tightened correctly and spoke tension is good, although the wheels spin freely, when under load something might be twisting or moving just enough to rub.
Check also for frame cracks - unlikely as Boardmans are generally good but there are people who think gravel is the new ATB!
Keep us posted, good luck.
