When trying to play with my fiance, she experiences a lot of stuttering. When opening the F3 menu, it shows that Minecraft is only using 512mb of ram, which appears to be the culprit. I had her open the minecraft launcher, go to the luanch options tab, and change the JVM arguments to include -Xmx2G, which didn't work. We eventually discovered that there was a system variable set that limited it to 512mb, which we deleted. We did this by opening control panel, going to system, advanced system settings, environmental controls, and then finally finding _JAVA_OPTIONS under system arguments. This worked at the time, but then we quit for the night. This morning, when we logged in again, she experienced the same stuttering issue. She checked her F3 menu, and it showed only 512mb again. I tried doing all of the exact same thing, but the system argument isn't there this time. I can not for the life of me figure out why it is doing this. Any help is appreciated.
EDIT: It's worth mentioning that she is running Windows 10, but using the Java launcher.
What I ended up doing was recreating the _JAVA_OPTIONS, but making the value 2048mb rather than 512mb, then restarted the computer.