Symptoms are as if wireless transmitter slows FTDI serial output,
which gets updated by USB before DMX frame has completed.
If possible, reducing channels is better than reducing frequency.
I suppose that your FTDI interface is recognized by QLC+ like USB Enttec Open.
Here are QLC+ DMX USB Enttec Open tuning parameters:
http://www.qlcplus.org/docs/parameterstuning.html
FWIW, I forget to look for QLC+'s User Documentation hyperlink on the right side of the main page
instead of at the top..