Slow sculpting on high density meshes

I'll add that if you have any vertex groups on the mesh, even if not active, when in sculpting mode, performance tanks. Removing the vertex groups helps quite a bit.

Make sure you're in Solid viewport shading, if you're on Material Preview it will be much slower.

Might be a detail, but took me a while to find out this

One of the major complaints with blender 2.8 right now is that, despite all the work and effort put into the sculpt mode, dynatopo, what-have-you, the vertex cap for sculpting is actually very low. My rig starts struggling at about 3 million polygons, and this is the same rig that can easily pull about 40 million on a Z-Brush sculpt. For that reason, I am using Z-Brush for anything I anticipate going over 2 million vertices. When I retopo, I also make dang sure to make sure the reference mesh is decimated to under 2mill as well, and use X-Normal to bake instead.

And remember, this is with 2.8, you claim to be using 2.79, which is before all the updates and speed improvements. As such, I am not surprised you are reporting the performance that you are. You can get a small speed boost by bumping to 2.8, but I would actually recommend looking elsewhere than Blender for the time being if you want to get very detailed.

but you are referring to lag when nagivating or when sculpting? dyntpo its better for having less polycount when sculpting but it affects the mesh and without a way to to delete it or undo what you done after finished the sculpt and also it works with triangules instead or quads. multires instead save this information in the modifier and makes a even subdivision in the whole mesh, but some times not necesary to have all the mesh subvided in areas that you wont sculpt, and it stays the original polycount and topology of the original low poly base mesh used. but you can use both like he does https://www.youtube.com/watch?v=Nc3cRVG3me4 also avoid to have matcaps and the outline select option checked because it will put it more slower when its very high poly.