Confused by vast difference in render time between two similar projects (.1 s vs. 120 s)

I’m still a beginner with Blender (been doing stuff occasionally for a year now), so I’ve got no knowledge about the intricate technicalities of rendering but this is the first time I’m really confused by render times.

My machine: i7-7700HQ (bench:, GTX1070 (bench:, 32GB RAM

So I’ve got two different .blend-files with heavy geometry: “A” has 1 mil verts, “B” has 2 mil verts, no simulations. Whenever I let Eevee render a 1080p/32samples-frame in project A it takes about .1 seconds whereas in project B it takes about 2 minutes (similar differences when using cycles).

My CPU is only working at 20%, my GPU is not being used at all. When rendering project B it’s stuck at 0% for most of the time, even becomes unresponsive, and then actual rendering is done in just a few seconds. So obviously there’s something going on while preparing the scene.

Now I’m really confused at why these two projects render at such vastly different render times. I think I’ve checked most of the relevant settings. The only major difference I see is that the geometry in project B uses 10 times the memory (6GB). Would this be the reason for the 2 mins of prep time?

Btw, everything’s running perfectly smooth in shaded viewport, but I’m assuming that has got nothing to do with my issue?