Hello,
It seems that you didn't create this report two times, there are no worries.
Unfortunately, there is no way to compare the Cloud SQL Postgre performance with another Cloud SQL service from my side.
It seems that you’re following the right steps to improve the Database performance, such as check the location, increase the tier when the instance looks throttled and upgrade the disk since larger disks have more IOPS available. I would add on your to-do list to implement cache, case your queries will not be constrained to I/O performance. Ideally, the entire data set can fit in 70% of RAM. Additionally, I recommend that you investigate non-Cloud SQL issues, like optimized queries by adding appropriate indexing, reduce the scanned data and avoid extra round trips. Uou can find some information regarding this in the following website[1].
You can troubleshoot performance issues by following the instructions in the following documentation[2].
I hope this documentation could help you to determine the root cause of this performance issue,
[1] https://use-the-index-luke.com/
[2] https://cloud.google.com/sql/docs/postgres/diagnose-issues