Seeing consistent serial data loss over USB, have ruled out hardware and rx/tx software. What's left?
The driver board for your "Lasersaur" appears to be based around the Adafruit Trinket Pro.

That board, on Adafruit's website, is deprecated, with the following warning:


  Deprecation Warning: The Pro Trinket bit-bang USB technique it uses doesn't work as well as it did in 2014, many modern computers won't work well. So while we still carry the Pro Trinket so that people can maintain some older projets, we no longer recommend it.


So basically the USB interface is naff. It's too slow to keep up with modern computers (hence your old laptop works but your new one doesn't), and you lose data.

You should look into the possibility of upgrading the controller board to something a little more modern and capable.



For UNO R3 based boards there is another fundamental flaw: there is a 64 byte receive buffer and no flow control.  That means if you send data too fast - faster than the board can process it - data will be lost.