Extreme 1TB SSD transfer speed very slow then spurts of fast

I have a SanDisk Extreme 1TB portable SSD, I use for backups of my computers. Normally I am transferring files of 50MB to 15GB and the disk transfers at a rate between 4-5MB/s up to ~250MB/s and take just a few minutes. Lately I’ve been transferring larger files up to 120GB. These have been transferring at a rate between 250KB/s up to ~340MB/s but staying primarily in the 200KB/s to 20MB/s range and the transfers have taken about an hour to complete.

I back up a desktop and 3 laptops, all running Windows 10 or 11, 64 bit. All are updated, scanned and backed up weekly. I am using the USB C cable that came with the drive with an adapter for two laptops with USB B ports.

I am doing the desktop now and it is running predominately in the 200KB/s to 18MB/s range. I used Task Manager to close as many unnecessary programs and processes as possible with no apparent difference. I notice in TM that there is a COM Surrogate process that is running at ~25% of CPU and rated Very High for power usage. I don’t know if there is any correlation to the transfer speed.

I’d love to see the SSD run at much closer to it’s rated speed of up to 400MB/s.

Thanks

… I’d love to see the SSD run at much closer to it’s rated speed of up to 400MB/s.

Actually, I just noticed, the Dashboard app that comes with this SSD says this disk has a transfer speed of up to 6GB/s! I’d settle for a 10th of that. Thanks

Can anyone out there suggest any fixes?