Minecraft lag with the AMD A10 7850K APU?

I've made my first custom build PC for my grandma. She doesn't really use it for gaming but her grandchildren do, and she didn't have much money to spare, so I've build a PC with an AMD A10 7850K without a graphics card, but with enough power to add one later. The PC has 8 GB of Corsair Vengeance RAM running at 2133 MHz.

The problem is that Minecraft lags at specific points, however, during normal gameplay it works just fine.

The Squid, for example, stutter instead of moving fluently. This only applies to the Squid though.

Also, I experience a severe FPS drop when it starts raining, but this only troubles me when I have Vsync enabled, however, turing Vsync off isn't an option.

When I have Vsync disabled, my maximum framerate is about 150 FPS, with Vsync enabled, it's limited to 60. When it rains and Vsync is disabled, my framerate varies between 80 to 100 FPS, however, when Vsync is enabled, My framerate drops to 30 FPS.

Finally, the game freezes for a moment when I set something on fire.

Especially the second problem troubles me, as my gameplay is impaired.

I've seen videos of people running Minecraft just fine with 1920 by 1080 resolutions, while I'm only using a 1280 by 1024 resolution, so higher framerates should be achievable.

Does anyone know what could cause these problems?