SanDisk Ultra USB 3.0 Flash Drive 256GB - Transfer starts fast then drops to a crawl

I bought two SanDisk Ultra USB 3.0 Flash Drive 256GB a few weeks ago from SanDisk via Amazon.

They were formatted as fat32 when they arrived so i reformatted them both as exfat because i am storing large iso files on them. When I try to copy a 4GB iso file to both of the drives it starts at 62MB/s then drops to 5MB/s then drops to a crawl 350KB/s and says it will take 24 hours to finish. I have changed the usb device properties to better performance and i have tried them in usb2 slots. I have also tried them on various machines i have ranging from old machines to brand new machines without any change. I have also tried ntfs with the same problem. I have also tried a few small files and they transfer fast, but a lot of small files and again a crawl.

Any ideas before i return them both?

New drives larger than 32GB are factory made as exFAT systems.  If your’s were FAT32 they are used.

After I wrote the post I started to think if they were fat32 or exfat when they arrived, but I remember trying to copy the 4.3GB ISO file and it failing saying that it can’t copy a large file, so they must have been Fat32, which is weird because they should have come formatted as exFat as standard because of the disk size.

I bought these directly from SanDisk themselves via their shop on Amazon so I don’t think they are used. They came in sealed original SanDisk packets with serial numbers, which I am going to try and check.

I read a few other posts about slow transfer speeds and someone suggested doing a full format rather than a quick format. Doing this changed the transfer speeds dramatically. The transfer still starts at 67MB/s but only drops to 20MB/s which I can live with, but this is nothing like the speeds SanDisk are claiming these drives can perform at.