What costs so much perfomance in my scene?

I have an animated scene with imported CAD models. Already upon importing the first model and applying materials to it, the perfomance started to decrease drastically, despite me ‘optimizing’ the models with a decimate modifier as far as possible to decrease the poly count. Naturally, this got progressively worse throughout the project with the poly count peaking at about 1.5 million. As an amateur, I can tell that it matters, but can’t judge how much this is for a software like Blender.

A week later with the animation done, I am now faced with the results of me working around the constant perfomance problems now that it comes to rendering it out.
Despite fairly powerful hardware, the individual images still take a solid 5-10 minutes to render each on relatively low settings.
I’m rendering with CUDA on a PC with a GTX 1080ti 11GB VRAM, AMD FX-4350 and 16GB RAM at 1024 samples in resolution 1920×1080 with a limit of 4 light bounces on Cycles.

Neither the models, animations, materials nor the lights in the scene seem performance-heavy to me, yet the file size of close to 900MB already shows that something weights heavy in there.
Even after trying to find the issue by deleting individual parts and materials, I couldn’t pinpoint any problematic objects and fiddled with the settings a lot.

At this point, I’m out of ideas what makes this scene so heavy on my PC and why it takes so long to render. Perhaps I’m just underestimating the perfomance impact of my models, but I’m hoping somebody would take a look at the project and maybe identify any issue there are.
https://drive.google.com/file/d/15GwUDALTaVzkN78s9mcnACfKXX25-T0k/view?usp=sharing

EDIT
Upon running more tests, I found a few things that did decrease render time signifcantly, but not to a degree where I’m reaching reasonable times.
First of all, removing the HDRI image and inserting a sun light instead has decreased the overall render time by almost a minute.
I have also managed to decrease the poly count to about 950k, but without any notable effect.
I also noticed that the largest part of the filesize comes from materials that were imported as placeholders as the start of the animation, but aren’t actually used in the scene and did not get removed from the actual .blend file.

Also, render time in EEVEE is about 3 seconds per image. While it’s clear that EEVEE is a lot faster than Cycles, this does seem a little excessive to me at first glance. Could be problem lie somewhere with Cycles?