I’d like to extend cordx’ reply by the following information and questions.
What were the presented reasons?
The point of using a live system and/or VirtualBox is to test out functionality and maybe compatibility. Not performance (I will explain, why.). I have a Laptop with 4GB RAM which allows me to easily run VirtualBox appliances to run with 1GB or 512MB RAM. You don’t need more. There is no need to test out an unknown distribution for playing YouTube videos or anything similarly RAM heavy, as this is something generally achievable and/or solvable.
If a live system is loaded into RAM, it actually could be faster than the installed version of that very live system’s underlying distribution.
Now, you won’t be able to “test out” performance as this is something too abstract to test if you try to reach a general conclusion out of it. Let’s say, you try out a system on by using your triple boot method. You install a fresh new distribution and play a YouTube video. Wow, it’s so fast. Great, it doesn’t lag. Your excited and you install the distribution seriously now. You use it for days, weeks, months. The more you use it the more the wowness and greatness depletes. Why? Because in this example scenario you loaded your distribution with additional software frequently which decreases the overall performance of your system. Is your OS “slow”? No, but your personal OS situation is.
The only performance tests making (slight) sense are when tests are done on a fixed environment with one or more fixed applications. That’s how game performance benchmarks are done, btw. Even those slightly valid benchmarks aren’t that useful in the real world, as your computer is probably differing from the benchmark environment, more or less. Usually a lot.
So back again to testing a YouTube video on a freshly installed OS. It will probably run great. Can you rely on this test? Probably not, as you probably will install more programs, use more programs, maybe even run more stuff in the background, all this while a YouTube video might play.
Therefore, testing performance before seriously installing and using a foreign OS isn’t a concern and can’t be a serious one, from an evaluation perspective.