I had a version installed, I think it was 3.4.10, and used it couple months back it worked great. I tried opening it yesterday and it took forever. I uninstalled it and installed the latest version 3.4.15 and that is also taking forever. I can't use it all. I uninstalled and reinstalled and still super slow. I see this is big problem but can't find a fix. Im on Win10 Pro.
Try to upgrade to the latest version.My suggestion is that you remove version 3.4.x and install the latest version 3.10.
From my experience QGIS loads slowly when we have too many plug-ins installed. Try removing those that you do not need.
Just to make sure that the issue is not related to your PC, try to install QGIS on another machine and see if the same problem occurs. Sometimes even antivirus could be the cause.