Minecraft does NOT require 6-7 GB - try lowering it to 2 GB, which I believe is the default, or uncheck the JVM arguments box so the launcher resets them to the default; your computer may be running slow due to lack of free memory, forcing it to page to disk (It doesn't matter if you have 16 GB, only what you actually have available, and the JVM itself uses a lot more memory than what F3 shows; a common rule of thumb is to never allocate more than half your free system memory - note that this is free, not total). You also seem to be using -Xms7G since the game will never allocate 100% of that much memory, at least not under normal circumstances (and especially not on a multiplayer server with a very low view distance) - you shouldn't use -Xms at all, only -Xmx, letting the JVM decide how much to allocate.
Also, always run separate versions in separate profiles and game directories; many game malfunctions and crashes are due to running different versions in the same directory, thus they attempt to read configuration files for the wrong version (this especially applies to downgrading, many older versions (at least) had no verification of settings and will even try to use settings that crash the game):
-game-directory/
This is also necessary if you are using different Java versions; if you can't find a Java folder in C:\Program Files then you need to install it from the official site (by default the launcher uses a JRE provided by Mojang, hence you don't need to install one; Mojang only supports the latest version so they wouldn't bother providing an older JRE even if it causes older versions to crash or bug out; this goes double for modded versions, which may require older versions of Java even if vanilla has no issues, largely due to a bug within Forge):
_manual.jsp