Hi, there could be a number of factors causing this.
The larger the brush, the slower it will be, this is unavoidable. The larger the stroke (in terms of number of dabs), the longer it will take to render, this is also unavoidable (though performance here could certainly be improved).
Turning the “Pigment” slider down to 0 in the brush options panel will turn off the new realistic color blending, but make the brush less laggy. Currently you need to do this whenever you switch brushes if you want to keep it off, or use the history panel to re-select the brushes where you turned it off.
Also, since you’re working on a laptop, the cpu will probably be set to run slower when the power is not plugged in, and in general it will try to “spin down” and use as little power as possible (to not overheat, waste energy, etc). This can also be a factor.
It’s very likely that in future versions the pigment mode will be a lot faster, but in 2.0 we will have to deal with the lag when using it. However, for smaller strokes and small to medium-sized brushes, pigment mode is usually not that laggy.