To my knowledge Revit rendering is CPU based so it is likely that your low powered CPU is the problem.
You can compare where your CPU sits relative to others HERE.
As you can see it's quite low compared to the more top end CPU.
One of the big issues you have experienced is that Revit 2015 used a different rendering system which has been removed for the new versions.
Personally I think the new renderer is crap in comparison to the old one and the 3rd party renderers that are available, but Autodesk don't like paying for quality things it would seem.......
What you can try is:
- Render dialogue box > Settings = Edit
- Render Quality Settings Dialogue box > Quality Settings = Custom > Light and Material Accuracy = Simplified > Ok > Render
This reduces the amount of calculations the CPU needs to complete so should help speed things up somewhat.
Unfortunately the modern world of Rendering requires a lot of power.
Upgrading your system may be the only option if you want to keep using Revit. (Dont, go use something else, your life will be more enjoyable)
Best of luck
