Extruding two edges of one of those surfaces leads to unexpected results.
We get one Srf and one PolySrf.
The flow of CP’s is irregular at few locations
And yes, lots of kinks involved here
Starting from this, it is not surprising that Extending such a surface takes some time to compute.
Based on these observations, I would not point Rhino 7 as the main problem.
I would instead investigate to try to understand which approach has been taken to obtain those surfaces in Rhino 5. ( possibly, in the context of the original RH5 file with all history relationships available things are more easy to manage… )
If the problem described here is related to the modelling approach, this may happen again no matter which version of rhino will be used…