Blender runnning slow in weight paint mode I have modeled a pretty complex object in blender and I wanted to rig it but then I noticed that in weight paint mode the viewport is painfully slow I checked if the CPU or GPU was bottlenecking the program but no the Ram, CPU and GPU were around 50 percent and the temp was around 50 Celsius and blender was still running very slow I could not even move the object without it lagging 
I have every modifier applied and I have merged some verticies (about 10000)
I can not figure out the problem
I am running Windows 10

blender 2.8 when I change the engine nothing changes
There are about 800 000 faces

I have Intel(R) Core(TM) i3´-6100 CPU 3.7 GHz
8GB Ram
and a Radeon 570 GPU (when I bought it cost around 250)
The lag is probably an indicator that Blender is trying to recalculate and redisplay one or more procedural modifiers while you're trying to weight paint.

Try temporarily disabling any rigs (which would cause Blender to recalculate mesh deformations to match the rig pose), particle emitters (recalculating position of all the emitted children), modifiers etc. that are using the weight paint.

Edit

If it's still lagging, your system may actually be the bottleneck. Many of the processes Blender must go through are single-thread processes that can't be spread out over multiple cores. Since 100% usage of a single thread/core is less than the total capacity of your CPU/GPU, it doesn't display as 100% usage.

You can confirm this by checking the individual core utilization in Task Manager. Look for single cores that are approaching max utilization.

Edit 2

You could also try using local view by pressing / to hide everything else in the scene. 

Your power settings in Win10 could be the issue. Open the start menu, type Edit power plan, go to Change advanced power settings, and check the Processor power management > Maximum processor state.