How big is the difference in performance if I were to use a 16GB RAM bar instead of a 2x8GB bar?

I installed two slots in my laptop and one of them is apparently defective because I can't insert a bolt into slot 1. Only slot 2 works and only if there's nothing in slot 1. If I insert a bolt in slot 1, the screen does not start when booting, but stays off. It's not the bolt, it's the slot and I don't know how to fix it myself. I don't want to send in for repairs and wait three weeks again, precisely because they always do a BIOS update there and thus everything is deleted from my laptop and then all the effort with reinstalling, etc. Etc. Unfortunately, 8GB RAM is not enough for anything off, it is usually 90-100% busy and I often get FPS drops when playing WoW, hence the question: does it make a big difference if I only use a 16gb bar instead of a 2x8gb bar and simply leave the "defective" slot empty? If so, I would send my laptop back in for repair.

You no longer have a dual channel.

Depending on the application, this can either make almost no difference in performance or a large one.

But usually it can be felt, at least when playing games.

But whether the cost of a repair is worth it is another question. If you still have a guarantee I would definitely do it.

Then take 16gb 1 slot better than a repair

I still have a guarantee, but that's not my point. I don't want to wait 3 weeks again because I have 80% free for the next 4 weeks and then I could neither work nor gamble during that time. It would be enough for me if the FPS drops would disappear, I don't play any games on the laptop that require extreme performance.

You operate a PC in dualchenel. Twice as fast.

So 2 times 8 extra than once 16.

You hardly notice that. This dual channel is overrated.
It also depends on whether a dedicated graphic is installed.