Minecraft uses all memory and runs really slow.

When i launch minecraft. It uses about 1.5GB on launch screen. When i then proceed to load a game or create a new world. my memory slowly increases to maximum allocated memory. (about 5 sec). 

When minecraft then reached it's maximum the FPS tanks down to the 30-60. This performance should not be happening on my hardware, when i then proceed to fly in creative in a direction the chunks struggle to render.

Whilst i can't seem to reproduce this myself. This is the 3rd time i've installed minecraft on a new pc and this is the first time happing. However i can link a forced crash log and some images

 

UPDATE: as of doing more testing i've found out that some modpacks like Feed The Beast Revelation (150 mods 120 FPS) packs dont experience this issue at all.

UPDATE 2: i've found out that optifine fixes the memory leak by purging unnecessary memory once it gets to high (Minecraft running at about 800MB). But the memory still increases, it just get purged when it reaches 1.5GB and falls back to 800MB, where it proceeds to increase again.   Whilst it fixes intense lag Minecraft doesn't run as well as it should compared to my SPECS (60-120fps)