Hey, thanks for reporting this.
It's likely because there's still just a ton of geometry in/around the city to render. I do remember that the train track columns weren't optimised that well (but didn't have time to fix that around launch. Sadly I won't be able to push any new build versions for a while due to other commitments)
Yes, certain sites in Act IV are designed with using the crane to access different buildings.
I know this isn't the best fix, but: reducing the resolution of the game helped my previous computers perform at rough moments. Can you try reducing the resolution from 1920x1080 to 1600x900?
- Peter