It takes up to 10 minutes to write file to GCS

That’s normal and I got the same experience. The machine type f1-micro is very very weak (that’s why it’s for lab purpose), so in order for it to complete everything written in the startup script, it has to "struggle" a lot hence took lots of time.

Try to change to "normal" machine types like n1-standard-x and you will see a whole difference in terms of performance. But that will be out of the "always free" tier and it will consume your trial credit, so better having a close look and terminate it when you’ve finished.