What I do: I create a scalp mesh from the head. Then I create vertex groups to control hair in different areas, mainly as the input for density. Weight paint at this stage is quick and responsive.
Fast forward a couple steps in the hair creation process. I want to adjust the hairline. Just as before I go into weight paint mode. However, weight paint mode is now laggy / unresponsive to the point where the cursor gets stuck for 10+ seconds.
Disabling objects/modifiers doesn’t improve responsiveness. Neither does deleting hair curves or modifiers.
Since the CPU has utilisation spikes in weight paint mode the second I want to do something with the scalp mesh, I’m sure Blender is calculating something. But I have no idea what it is, where to find it or how to temporarily stop Blender from doing that.
Duplicating the scalp object solves the issue; weight paint is responsive again. Separating all the hair curves from scalp object #1 and then attaching them to scalp object #2 works as well. It doesn’t seem practical though; editing scalp object #2 becomes just as unresponsive as #1 after doing that, so every change to a vertex group in wp mode would require me to repeat the whole process. I’m pretty sure I’m missing something rather obvious here 
Any help appreciated <3