Why am I seeing low transfer speed with Robocopy between two external SSDs? [duplicate]

Question

I'm using Robocopy within a PowerShell script to backup my main SSD to another SSD, but I'm having problems with the file transfer speed.

While in Windows Explorer I have writing speeds of 300~400 MB/s, in Robocopy the average is 80 MB/s.

My command line is this:

robocopy "$($source = $drv;$source)" "$($destination = "$dst";$destination)"  /e /MT:4 /NJH /NDL /NC /BYTES /copyall /A-:SH /xd $IgnorarPastas /xf $IgnorarArquivos /log:"$dst\_Backup Logs\$filedate-robocopylog.txt" /tee | Get-RobocopyProgress -ErrorAction 0

The reason I'm using Robocopy within PowerShell is because I created a script to detect the backup and destination drives automatically, this makes the process simpler and more visual.

Another point is that I use a function to have copy progress with PowerShell.

What I've already tried.

1. I have already tried to change the number of cores by changing the /MT parameter

2. I've already tried disabling the progress function to see if it was interfering. I got the same speeds.

3. I've already tried disabling the log, same speeds.

Considerations:

1. Both SSDs are Corsair MP600 Core.
2. The SSDs are external and are connected by Orico enclosures, one has a 40Gbps Thunderbolt interface and the other has a 20Gbps USB 3.2 Gen 2 interface.
3. One of the SSDs uses my notebook's Thunderbolt 4 interface, the other is connected to USB 3.2 Gen 1. I've already tried reversing their connection.

Comments

Analyzing the behavior of the SSD in the task manager, I see that the SSD that is receiving the backup has 100% usage and a high average response time, around 2000ms.

Any ideas on how I can resolve this?

Edit - Additional Information

Initially I thought it was something related to Robocopy because I don't usually copy large folders directly in Windows Explorer, so I wasn't seeing this oscillation since the copy speed remains high for about 10 seconds, which is often enough to accomplish copying the files.

Now I did in-depth tests, copying large folders to be able to analyze the Explorer copy process for longer, and the result is this in the photo:

In the graph it is possible to observe the speed oscillation, it remains for a while at around 400MB/s, oscillates, and then remains stable at around 80MB/s.