Can you put a x16 GPU in a PCIe x1 slot, and will it work? It depends. There are several reasons you may want to do this, but you may run into some issues.
A x16 GPU functions just fine in a PCIe x1 slot. However, in most motherboards it doesn’t fit unless you modify the card or the slot.
Why you may want a x16 GPU in a PCIe x1 slot
There aren’t a lot of GPUs that fit in a small form factor desktop. And many SFF desktops, such as Dell Optiplexes, only have two slots. A single-slot card like a GT 1030 fits into an SFF Optiplex just fine. But if you want a higher-performance card like a GTX 1650, it doesn’t fit in some models like the 3020, since the x16 slot in a 3020 is right up against the power supply. You’d have to put it in the x1 slot instead for the double-wide card to fit.
That doesn’t fit either, so you’re stuck modifying the card. Or you might consider moving the Optiplex SFF board into a different case. That’s difficult too, but less risky. But that’s just one use case, so if you’re here to talk about using an x1 slot, let’s talk about using an x1 slot.
Modifying the parts to fit
Some PCIe x1 slots have an open end to plug larger cards in. Most do not. That means you have to either cut a slot in the card seven positions after the existing slot, or cut the PCIe slot so the remainder of the card can hang out. If there’s enough clearance, I recommend modifying the slot. I’d rather risk ruining a motherboard than a $150 GPU. Used SFF motherboards are usually really cheap because there’s so little demand for them.
To open up the end of the slot, some people use a soldering iron to melt it away, and others use a rotary tool. The soldering iron is certainly cheaper, but the rotary tool limits your exposure to toxic fumes from melting plastic. I don’t recommend the soldering iron.
If there are components in front of the slot, you may have to cut the connector on the card. This works, but I’m not exactly a fan of permanently modifying a $150 graphics card. If something goes wrong, you’re out quite a bit more. Leave seven fingers on the card to the right of the slot, then cut the remainder of the PCIe connector off. Of course I can’t be responsible for any damage from doing this. I don’t really recommend it.
If you have room in your case for it, you can get an adapter or riser, which consists of a PCIe x1 connector for your motherboard, a flexible cable, and a x16 connector for your GPU. Then you plug the riser into the board, position the GPU somewhere you have room, and secure it with a screw. This permits you to use the GPU in an x1 slot without modifying either your board or the card. If you have a micro ATX board in an ATX case, this is a fantastic option.
The performance hit from using a x16 GPU in a PCIe x1 slot
Performance takes a hit from using a low bandwidth slot, but not the 93% hit you might expect from using 1/16 as much bandwidth. The performance hit ranges from 7% to 40%. This also means you’ll get better performance with a GTX 1650 than with a GT 1030 in a SFF machine, even if you have to put the 1650 in an x1 slot because of the slot positioning.
A better option: Get an x1 GPU
They’re harder to find, but you can get an x1 GPU. Make sure it actually has an x1 connector on it. You won’t have as wide of a selection, and you won’t be able to get something in the GTX 1650’s league, performance-wise. But if you need a card to add additional monitors and an x1 slot is all you have, I recommend looking for an x1 GPU.
If you need something resembling modern GPU performance and your x16 slot butts right up against the power supply like in an Optiplex 3020, I recommend settling for a low-profile, single-wide GT 1030. It’s half as fast as a GTX 1650, but it fits.