Look up your owners manual and go to the section on the pcie/m.2 configuration. See what slots run at what speeds, and use what pcie lanes.

Your CPU has X pcie lanes and you're able to use all of them. Your chipset also has X pcie lanes, but some are used for the USB ports, some for sata ports, etc. If you put the SSD into a slot that's already maxed out on pcie lanes then it's probably going to drop the lane count of multiple things down (the process isn't super smart).

My desktop has the two x16 slots connected directly to the CPU, and 2 of the M.2 slots are connected to the CPU. If I install just my GPU and just my main SSD I run at full speed, but if I add a second M.2 SSD my GPU drops to x8 speeds. I think there's a 3rd slot hidden away that's on the chipset.

Do the slots share lanes?

If you remove the last drive and try again is it any faster?

My motherboard has two M.2 slots, but they share a PCIE gen3 lane.

If I put SSDs in both, and they are both NVME, (I could put a sata SSD in one of them) they both drop down to gen2 speeds as the lane gets bifurcated in order for both slots to work at the same time.

In my case, my mother board manual actually DOESN'T mention this, so I had to find out the hard way when I plonked in a second drive. I did check when I bought the mobo because I knew this is common, but oh well.

What's the make and model of the things?

Which os are you running?

Try to partition it with free space at the end and see if it makes a difference.

Try to trim the drive and see if it speeds up again.

Do you use any disk encryption?