Is this a clean install of Windows? If not then that's where I'd start.

Just remember if you format everything you lose the recovery partition. That said, I generally prefer a clean install and don't use factory recovery partitions anyway.

If a fresh install of Windows doesn't resolve the issue then I would wonder if you have a hardware issue. Since it sounds like the motherboard may have been replaced there could be faults or it could be underperforming because something is overheating. If the wrong thickness of thermal pads were installed or some are missing that could cause thermal throttling.

I doubt anyone swapped the GPU. That would be a huge undertaking. More likely the whole board was swapped by a previous owner.

Keep in mind that benchmark leaderboards can be skewed by different cooling configurations and power delivery. Someone might decide to open a laptop up and Frankenstein a massive cooler onto it just for fun. Sometimes someone gets a unit that runs super efficient or one that barely makes the cut. Or sometimes a simple driver/firmware update changes how power and cooling are balanced. I would lean towards review content and see how you stack up against the reviewer's experience rather than chasing benchmark numbers.

But to what I was saying, thermal pads come in thicknesses varying by .5 mm and using the wrong thickness can cause heat pipes in a laptop to not sit properly. Sometimes a manufacturer might use more than one thickness as well so the wrong thickness on one set of components could cause the pads elsewhere to make poor contact. I don't have specs for you as to what the correct thicknesses should be for your machine but it's something to keep in mind. If you find that some temp readings are good while others are hot that could indicate a problem like this.