First, please keep in your mind this is my subjective opinion, based only on my own experience. Facts can be found on the net and in the library
I have had no speed or other problems in using different Linux distros in VirtualBox, or installing a Windows / Linux dual boot. I guess it's partly because I'll always try to do things, especially when I'm doing it first time ever, exactly as I've been told and / or how the manual or instructions say it should be done. I mean, no shortcuts or "that must not be important, I'll just ignore that" attitude. Like everything else in computing, the principle is easy: If your system is well maintained and you follow the instructions, it usually works.
If the Guest Additions are installed, RAM & Video RAM memory settings correct, there should be no speed problems. In any case you are not using a virtual machine to play GFX intensive multiplayer online games or to edit HD video.
If dual boot is more your thing, I recommend using BIOS to choose which OS to boot, if your system is up to it. If you use traditional bootloader, my recommendation is and has always been to install Windows first, and let Linux handle bootloader.