One bike for road/gravel riding

[cougie uk]

I have 700x28 on my road bike and 26x50 on my touring bike, both with road treads.
My average speed over a standard 56km local route is the same on both bikes, possibly 0.5km/h higher on the touring bike.
The touring bike has a Rohloff hub, the road bike has 1x11 derailleur gears.
The touring bike is heavier, but it is more comfortable, and not any slower.

So, yes you would notice the difference on the road between 25mm slicks and 44mm tyres but not necessarily in the way you might think.

That's two completely different bikes though. Weight, position, equipment differences would make comparison of tyres pointless.

Have one bike and change the tyres completely and it's very noticeable.