I have this same issue on my iPhone 14 Pro Max.  The Flyovers begin at a nice smooth frame rate, but like the OP stated, the frame rate progressively gets worse until it’s chugging so badly that it becomes unwatchable.  The frame rate used to be smooth for the entire duration of the Flyover a few months ago so I know this isn’t a lack of hardware power. 

Same is happening on my iPhone 13.
For Activities over 15-20K the flyover starts to stutter.
I tried to record a flyover for 120K ultra and after 30K it’s unusable.

Hello and thank you to everyone who posted here.

We are aware of this issue and are currently investigating. I will post any updates here when they become available. Sorry for the issue and we ask for your patience while we work on getting it fixed.

Hello Everyone,

I’m going to mark this post as resolved as we haven’t received any additional comments that this is still an issue. If you still have trouble regarding this issue, please submit a ticket and we’ll be happy to investigate.