Novel embedded and ubiquitous infrastructures are being realized as collaborative federations of heterogeneous systems over wide-area networks by means of publish/subscribe services. Current publish/subscribe middleware do not jointly support two key requirements of these infrastructures: timeliness, i.e., delivering data to the right destination at the right time, and flexibility, i.e., enabling heterogeneous interacting applications to properly retrieve and comprehend exchanged data. In fact, some middleware solutions pay more attention to timeliness by using serialization formats that minimize delivery time, but also reduce flexibility by constraining applications to adhere to predefined data structures. Other solutions adopt XML to improve flexibility, whose redundant syntax strongly affects the delivery latency. We have investigated the consequences of the adoption of several light-weight formats, which are alternative to XML, in terms of flexibility and timeliness. Our experiments show that the performance overhead imposed by the use of flexible formats is not negligible, and even the introduction of data compression is not able to manage such issue.