There are a few free copy tools that allow you to throttle the copy speed, here are a few suggestions:
1. Robocopy: This is a command-line tool that comes with Windows and has a /IPG (Inter-Packet Gap) option that allows you to specify the wait time (in milliseconds) between packets. This can effectively throttle the copy speed. Here's an example command that copies files with a 100ms delay between packets: `robocopy source destination /IPG:100`.
2. FastCopy: This is a free and open-source file copy tool that allows you to adjust the copy speed. Simply go to Options > Transfer > Speed > Custom and set the desired speed.
3. Ultracopier: This is another free and open-source file copy tool that allows you to limit the copy speed. Go to Advanced options > Speed > Limit speed and set the desired speed limit.
4. Copy Handler: This is a free file copy tool that has a "Throttle" option. Go to Settings > Transfers > Throttle and set the desired speed limit.
Hopefully, one of these tools will meet your needs for a free copy tool with throttle capabilities.