If Mixbus takes a very long time to start-up in Windows, then there are 3 known issues that you might investigate:
If you have ASIO4ALL on your system, then it will double the time required for scanning, because when Mixbus asks ASIO4ALL what sample rates it supports, ASIO4ALL then has to query all the devices (again). Mixbus can open MME devices directly, so there is no need to use ASIO4ALL with Mixbus. But you might need ASIO4ALL with other software.
Similarly, if you have JACK installed on your system, then it will double the time required for scanning, because when Mixbus asks JACK what sample rates it supports, JACK then has to query all the devices (again). If you also have ASIO4ALL, then JACK has to query that for the available soundcards... and you can see how this can exacerbate the issue.
The third issue is the Windows built-in GS Wavetable Synth. Some users report that the startup is dramatically improved if they uninstall or disable the Windows GM synth. Check Windows's "MIDI Device Window" to see if this is enabled on your system. The setting varies with each version of Windows. Unfortunately we are not experts on this subject, but users have reported that this causes a dramatic delay in startup when this device is enabled.