Hi @part_timer!
Ok, so lets gets some details clear first and then to an answer and solution! 
Not long at all, actually. At least one command is replied to per main loop, which on average are at most 1 ms. Therefore, the processing time for commands is pretty low (µs). The transfer rate of the reply is directly linked to the baud rate, of course. For example, a reply of *0QD300\r has 8 characters and would take about ((1/(baud rate / 10)) x 8) s to send, or about ((1 / (115200 / 10)) x 8) = 1.0850694e-5 s => 694.4 µs.
If you want more details on how long the entire process takes (sending, processing, replying), please have a look at this post. I go in details about the protocol and how it is handled and that includes logic analyzer screenshots of actual communication with a PC (USB) through a LSS Adapter Board and a LSS.
As you can read in the topic linked to above, many things can get in the way of obtaining a response. Most likely the size of the data payload and hardware FIFO are most likely responsible for the delays you see (combined with drivers, OS, etc.). On Windows 7 & 10 I’ve routinely seen ~15 ms (or 0.015 s) delays and sometimes up-to ~100 ms, depending on what else is happening simultaneously (and, of course, sometimes much higher but those are quite rare/abnormal).
I hope this info helps. Let me know if you have more questions!
Sincerely,