Noticed this when I was trying to transfer multiple files totaling around 30 GB one shot from my 15" M3 MacBook Air to my Samsung T7 shield SSD (AFPS) yesterday. It was slow at around 2 MB/s for the first 10 minutes I think then later the speed picked up so I decided to do a speed test today.

I have 2 other 2 TB (4.5% and 54.4% filled respectively) T7 drives and they did not have this problem. Both average 800 MB/s write speed and 600 MB/s read speed. Only this 4 TB (87.5% filled) T7 drive is giving 2 MB/s write speed. I also did not have this problem when using it with my old 15" M2 MacBook Air previously.

I did a 11 minutes test with 1 GB file size on my 15" M3 MacBook Air with the included USB C cable and most of the times, write speed is around 2 MB/s. Only a few times (3:38, 8:17, 8:24) it went to around 800 MB/s. I don't know if this is due to my drive being filled to more than 75%. I will try to move some files to my other 2 drives and retest.

If you have the same problem, kee chiu!

Update:
Fred the drive to less than 75% filled did not solve the problem. But I noticed the slow speed is for first few minutes and then it will consistently be around 800 MB/s. Once I remove the drive and plug in again, it will be back to around 2 MB/s again for a few rounds.