Is there a data transfer limit for virtual serial ports
This is explicitly a "X-Y problem". You have a typical audio application. To implement the real-time data processing for the signal, you have two basic options:


Implement the most trivial and common and easy USB class - CDC (aka "virtual COM port"). To accomplish the overall data processing goal, you will need to invent a method/format to pack the electret mic ADC data into UART-type stream, then use a common Windows/Linux COM-port driver to buffer and store the data, and then to develop a proprietary application do deal with your proprietary format. The plus of this approach is that you control the raw data format and don't need to dig any specifications for the data stream. Minuses of this approach is that you will need to develop a lot of your own code. 
Since this is an audio device, the formal solution is to implement the audio-class USB device within the Nucleo board. As I understand, there are code examples form STM development, see USB Audio device class on NUCLEO-F446RE and USB Audio device on Nucleo F446-RE with CubeMX. Advantages of this approach would be all well-established libraries in all known operating systems. Minus side is that you need to dig into audio specifications.


In all cases the bandwidth of USB (even in FS mode) is well sufficient for audio processing tasks.