So, I'm making quite a big mesh and at some point I've reached 1.5-2 million verts. This, obviously, slowed down Blender and it's near impossible to work. Is it ok if I turn off Dyntopo? If so, how do I make sure that the part of the mesh I'm working on is at enough high detail level?
Lets be clear: If a feature isn't working, don't use it. But I'll also provide an alternate perspective.
Z-Brush only recently got its Dynatopo equivalent: Sculptris mode. The workflow for Z-Brush, which has been the industry standard for years, is to make a lowpoly base mesh and sub-divide it until there is enough density of vertices to keep the detail. The artist usually is working with a mesh that is at least 3 million polys at this point.
What I'm trying to say is: it is ok if you are starting with a high vertex count and work from there, it's an established pro-workflow. It is a shame Blender isn't able to handle larger poly counts right now, but that is out of our control.
Remember that while in Dynatopo, the size of your brush will affect the vertex density. This means you can remove verts by making your brush larger. You can reduce parts of the mesh that don't need the detail so you can redirect that budget for parts that do.