why is minecraft lagging?

[Blastcakegaming]

so i have a custom built pc. here are the specs:

CPU: AMD fx6300
GPU: sapphire r9 280 (overclocked)
Ram: kingston hyperx fury 2x4gb

thats probably all you need to know.

so before i used to get something like 158-180fps in minecraft.
the when i got shaders mod with a texture pack, i get something like 20 which i dont think is normal. when i disable both shaders and the texture pack, i get something like 50 fps!
also, i set minecraft to use 6gb of ram but it only uses up to 2gb.

so i still can't find why this is happening? by the way it only uses 20% of gpu and 25% of cpu

 

[alex_was_here]

Make sure your power plan is running in high performance mode here is an article on it.
http://www.tenforums.com/tutorials/2843-power-plan-settings-change-windows-10-a.html

 

[ModMonster]

Are you using the Optifine mod? Optifine allows the your CPU to use more than 1 core. I have a 750 Ti and can easily get 300FPS at maximum settings at 1080p.

 

[l33tabix]

other than the PSU being to low of a wattage to overclock or there is any software problem Minecraft should not run that low on your system. Maybe try a virus detection program or to cool your pc down a little.

 

[offroadguy56]

One thing I learned, and this probably isn't the problem but it's worth checking out, is that you can tell minecraft to use 6gb of RAM but if it doesn't need that much then it won't use that much. But what I wanted to mention is that if minecraft is using the 32bit version of Java then it won't use more than 3.5gb (I think) of RAM.