is it the bike or the rider?

Genuinely...I believe between 2-3mph or so depending on the bike.

That's my experience over the last 8 years on a variety of bikes. If you like to put the effort in you will be rewarded...if you're more of a cruiser then a lot less so unless we're talking hills.

Also I'm assuming you're comparing your heavy, cheap bike with something light, stiff and racy. It'll also be a lot more fun to ride in my experience.

Yes...it is about the bike! (Within reason, sometimes )

I think so.
I rode an $18000 tour bike during summer. Wow. Super light, super smooth, highly efficient. Even wheel changes were super quick when needed with a brake cable tension release button on brake lever assembly.
I ride a bought used alloy Avanti Blade. Build cost $600Aus (300 quid) which includes purchase of bike.
Using microshift 9 speed to get durability of a nice thick chain (that's what I was hoping).. Doesn't compare to shifts on modern shifters but I like it. Nice and cheap. Holds ratio. Good enough if into picking a ratio and maintaining use of. Sucks if into constantly shifting to maintain cadence, acceleration in a pack.
I use as a gravel bike but even if I put aero rims and skinny tyres onto that. No way that will catch that. No way..
I don't care though.. Happy cyclist too bro.. Fast enough

A 1.1L Corsa can keep up with a Porsche if the Porsche isn't trying to go fast. 

What is the question?

In a context of non-competitive cycling, body position and tyre choice/setup will make almost all the difference. The rest is just noise. 
Also (virtual) GP5000s in 35mm would be faster than 25mm Gatorskins so let's don't compare apples to oranges when it comes to tyre width. 

I have been investigating this question for a few weeks- using Strava and Fitbit, I have so far loosely  established that over 1-2 hours of hilly cycling in N Lancashire, a 9 speed triple Merlin titanium with 23 mm tubed tyres, stiff SPD shoes, standard twin caliper brakes is only 5% faster than my 9 speed double Vitus Substance steel framed gravel bike with excellent twin piston TRP Spyre brakes, 38 mm tyres with tubes, bendy SPD shoes

I once heard a fella say that it's not about the bike. He was 100% correct.

Quote:

How much faster is say a 2-3 grand road bike than a halfords road bike?

Difficult to say, I'd guess maybe 20% quicker as a ball park figure.

If you can borrow or hire a more expensive bike and a power meter you could measure the difference to get a better idea. Like this - https://road.cc/content/blog/175749-buying-watts-how-much-difference-doe...

It's the rider.

hmmm, so are you saying you were racing them and asking if you had a better bike could you have beaten them?

Well they don't look like they were trying too hard for one, it looks like you were the only one racing plus in heavy traffic like that you cannot ride full gas and so cannot compare bikes. If you really want to know the answer, then enter a real race or time trial, then hire a better bike and try it on that.