The combat is rather fast-paced, yeah.

You could play on the lowest difficulty to mitigate some of it through lower damage taken, but it's really hard to say if it would still be too quick.

A playstyle relying a lot on the Quen sign might be an idea, but, again, I've no idea if it would work for you.

You wouldn't happen to have a friend or relative who owns the game and would let you try it to see if it's too quick?

Not that difficult.
It's mainly about dodging and not getting surrounded. Which isn't difficult, because almost all enemies telegraph their attacks clearly. When pressing dodge button, Geralt will automatically adjust direction of dodge. Roll lets you evade sweeping attacks from large enemies, but also quickly get out if you're getting surrounded. You can also roll under arrows.
Then you have witcher magic, Signs. Quen can fully absorb any impact, Aard staggers enemies. They're mainly support tools, though it is possible to create builds using them as main weapon.
You can also buff yourself using alchemy, which is almost too convenient to use. You're only limited by your toxicity limit, which even completely stock allows for buff yourself with 3 to 4 potions at once. Potions automatically refill when you meditate.

Worst case, you can play on Story difficulty where you can win 90% of fights by spamming light attack button.

First of all, I am a big witcher fan who has spent hundreds of hours in this game and I love it. However, the biggest problem I have with Witcher 3 is it's easy difficulty. You can even start with the hardest difficulty setting (deathmarch). Just remember to use left ALT when you see enemy attacking you. It makes Geralt dodge the incoming attack and doesnt cost any stamina. You just press ALT and avoid damage. The other overpowered thing is quen sign which totally blocks enemy's attack but breaks after sustainig damage, but can be cast again and again. It's generally very easy game. Why do I suggest playing on the highest difficulty? It's simply more immersive and you will get much more experience from the game, as it forces you a bit to use alchemy and crafting. Trust me on this one. It could be challenging just at the beginning when you haven't grasped the controls yet. Then, its quite straitforward.

Lowest difficulty if you don't want it to be too bad.

Otherwise, you are meant to invest heavily into stuff that gives you an advantage, even on normal difficulty. The game gives you tons of potions and a way to easily replenish them all instantly whenever you rest for a reason, you are meant to use them liberally.

The combat is decently fast paced, though thankfully you can give yourself a generous safety net with "quen" which essentially negates all damage up to a certain extent.

The point being for all the difficulty, there are options to negate it.

Difficulty can be changed at any time, and has no lasting impact on progression, so OP may as well start with Story difficulty and increase it as gets better grip on combat and game mechanics

roll, roll, roll , roll... roll some more lol

Thanks everybody for your responses. The general consensus is that it is very easy. I had a look at some gameplay and the fighting does look a bit overwhelming. It seems like hotkeys are needed which can be learnt but while I enjoy an immersive game play and a good story I don't want it to be too stressful or me mashing buttons quickly. I need to look up the quen button as I have never heard of it.

I am 76, been a gamer from day one (being into IT really early) and I also worried about my slowed reflexes. It is not a problem, spam the key, leap about and you will look and act like a cat on steroids. Just finished the game, ending is truly awesome. Good Luck.

78 here....took me a while to get reasonably familiar with the controls...really had issues for a while...now much better. It's a learning process...you make what improvements you can here and there...pretty soon you do much better. I concentrate on improving weapons and armor...and learning what you can get away with...make a mistake...try again.

You can only do what your level allows...can't force progress.

I'm having "vision issues"...can only play just so long before my eyes bother me...running an RTX 4060ti using Geforce recoed settings.