since minecraft uses its own portable java version. this happens:
computer bootup: start java
minecraft bootup: start java(portable mc version)

this causes java to be active twice. the only thing i dont understand is
when "mc java" uses 10% CPU
other java uses 10% cpu as well.
this java is used for the other stuff on my computer that uses java.
but this bug causes my "gaming pc" to be a half "gaming pc". efectively changing it to a regular 4gb ram intel i3 processor pc.

temporary workaround: go to task management after starting up mc. click the "proccesor" tab untill programs with the most CPU used are on top. you should see 2 java platform se binary programs running. one has an arrow before the name. if you click the arrow "minecraft 1.8.X" should pop up as a sub program. the other one should not have the arrow. right click the one without the arrow and click terminate to stop it.