One way to speed up startup is to disable the auto DFU/firmware update functionality. This is done by commenting out
USB_USART_LineCoding_BitRate_Handler(system_lineCodingBitRateHandler);
in system/src/main.cpp (currently line 199.)
I’ve added an issue to our backlog to make this configurable at runtime, since it’s mainly a development tool for quickly loading firmware via DFU/Serial without having to touch any buttons on the device.