The commands and the tools you are using are perfect for debugging problems in physical disk, the problem here is that the structure in the cloud is completely different. A Persistent Disk on GCP is not actually a real disk-- It is a Virtual Volume that uses a lot of Physical devices. And those devices use the Google network and other structures to work.
The following vide explains better how it works:
So, there are many factors that determine the performance of the Storage Volumes.
According to the official documentation, you can review persistent disk performance metrics in Cloud Monitoring, Google Cloud's integrated monitoring solution.
You could check this other document that can help you to check your disk performance.
If your workload has a bursty I/O usage pattern, expect to see bursts in throttled bytes corresponding to bursts in read or written bytes.
Databases are a common example of bursty workloads. Databases tend to have short microbursts of I/O operations, which lead to temporary increases in queue depth. Higher queue depth can result in higher latency because more outstanding I/O operation requests are waiting in queue.
Per all the systems involved in the Cloud for storage, If you want to know with more details about what is happening with your performance, I recommend you contact GCP Support, they should have more tools to troubleshoot your issue.
If you have a Free trial account you can have chat support through Console Support Center, also, you can visit the following link for more information.
Or, you could contract a Support plan in order to have cases for technical support through phone, and chat.