Using the Chrome developer tools with my tablet connected to my pc I was able to do some debugging.
Apparently Mendix adds Xpath constraints toall entities to not send too much data to the tablet. However, these XPath can become quite large and time consuming, in our case resulting in a time-out.
The User could navigate to a flow where a client name was displayed. On synchronization, all clients which could be retrieved by any path would be sent to the tablet. We have multiple paths leading to the Client entity resulting in a huge XPath with multiple or statements.
I solved this by copying the Client name to a new entity only available in the offline part, minimizing the retrieval time and preventing the time out error.
This issue should no longer occur in Mx 7.11.
"We simplified data synchronization for applications. Previously, we tried to filter the data by analyzing your model, but as models get more complicated, this approach became complex as well. Now, all instances of entities used in offline applications will be synchronized to the local database. "