Update: thought of an alternative solution. So far my approach has been to add MKPolylines each with its own renderer. But then I thought: "what if the amount of polylines is not the issue but the amount of renderers is?" So I tried to change my code back to adding standalone MKPolylines with a single renderer for drawing but that didn't work. Had the same problems as before: map/app freezing and/or crashing. I'm sticking with the MKMultiPolyline and MKMultiPolylineRenderer for now.

It depends on what you have to draw I suppose. If you have hundreds of polylines with different shapes and colours then yeah, there's definitely a problem. I didn't have that many individual renderers before. I also remove overlays when the user moves the map and only draw the polylines within the visible map region. But since moving my code to MKMultiPolyline with a single renderer my app feels a lot faster.