My mistake. I was trying to understand why you’d be using the Paint tool rather than an art program. If you add some paint to a layer, then leave the layer and take a look at the effects controls panel, you’ll see every brush stroke is adding as a separate effect. And each of these has a number of settings. My guess would be that you are using up all available processing power as you paint in much the same way as if you had dozens ror hundreds of Lens Flare effects on one layer.
I think other software might give you a better experience for painting and like I mentioned in my other reply, you can then export from these and bring the result into AE to finish.