Hey Augusto!
Welcome to the Memex community forums, hope I can help you further. 
I need to way 15 minutes to have a recent entry in a particular device/browser to be replicated through all my devices. Am I correct?
For now the backup is only a backup, not a sync. With the backups it downloads an update of your changes every 15 minutes either to your local hard drive, or Google Drive.
We are about to launch our first version of the sync between devices, initially limited to one desktop extension and a mobile device. This will be subsequently be expanded to 3+ devices that can then also be other desktop browsers or profiles. There the sync happens every 5 minutes initially and we will likely make it real-time as soon as we have the infrastructure cost-efficient enough.
I really like the idea of offline content but it would not be easier (and user-friendly) to have a cloud service for that?
Yes indeed it would be more efficient, and user friendly, in the short term.
With Memex we are all about data ownership, and so we are developing our software in a way that starts with data being in the user’s hands by default and gradually introduce cloud components that a user can use dependent on their use case. Because I agree that for some situations (like sharing content with others, or web-accessible knowledge bases) a cloud has many advantages. But for us it is important that those are choices the user CAN make. They should not be forced to upload all their history and private notes to a server by default.
Our approach of developing with an offline-first approach indeed brings UX and development challenges, like syncing/merging of data across multiple devices, slow update cycles. It is the harder route, but we believe it will pay off in the long-term because it creates a service where the user has the choice of where and how their private data is processed, stored and shared. It’s one of the fundamental pieces of our vision that you as a user have this control and can migrate to services that suit your needs better, or just play with your data in whatever way you want. That would be much harder with a cloud first approach due to cost, and infrastructure hurdles.
