It looks like the Windows Defender Real-time protection can cause issues when trying to copy files on a local drive even if the drive is an SSD. Typically a copy seems to start out great, but then slows down to a crawl less then 1MB/s copy rate. Probably has to do with Defender having to scan every file as it is copied.
To fix/increase the copy speeds you can turn off Real-time protection.
To turn off Windows Defender Real-time protection, open up Settings > Update and Security > Windows Security > Virus & threat protection “That should open up the Windows Defender Security Center” > Virus & threat protection settings
Now Turn Real-time protection off.