Hello Ivan,
I definitely agree that this should be faster. It doesn't look normal and the documentation also mentions that it should be faster, however there could be other factors affecting this.
I tried to reproduce your issue and I used a 60mb tar.gz file instead (half the size) and I did not have the same results. The extraction finished within a second. 6 or 8 minutes as you mentioned is not normal since that is slower than certain home hardware/connections and it's a small file. I suspect that this may be related to your project environment or the file itself somehow since I couldn't reproduce this (or maybe something else that I can't perceive at the moment). Typically, if I was able to recreate this, I would advise you to report the problem in the issue tracker here[1]. However, in this case it would be best to get in touch with one of our support reps if you have a package[2], as it would be more suitable for this kind of specific question.
If you do not have a package and you believe that I've missed something or that this is reproducible, the issue tracker[1] mentioned before still applies. The issue tracker also allows you to search for similar known issues. Hopefully this puts you in the right direction.
Sincerely,
Alexis
Google Cloud Support