Is it possible to calculate the data with FastLED or something else on the computer, send it to Arduino and let it display instead of sending the uncalculated data to Arduino and let it calculate?
Yes. I use Processing to do that. The Processing program does the calculations and sends the data to an Uno through the serial port. Robin2 uses Pyhon to communicate via serial.
A great tutorial to get started with serial communication.
The software on the PC is out of scope for the Arduino forum.