I see this complaint more often about the Zyxel routers odido is using, I don't think you'd really be able to improve on it. If you really need fast network storage, you're better off getting a NAS. if its just about storage you could probably get a cheap 2nd hand and be just fine, just make sure it's still supported with at least security updates by the manufacturer. 

It is not advertised to use the router as file server. For small logfiles it is ok, but it can not replace a normal NAS.

Thank you both for your replies. Definitely, NAS is the way to go. My expectations are not similar to the ones of a NAS, I just noticed the big difference between read and write. I believe the read speed is decent