OK, I get it now. Quite surprising that serialisation is a key performance issue. But then again, pushing signals in and out of CAN frames is also a key performance feature in regular ECUs so I guess that's a universal issue with all sensor/control/actuator systems.

Finding the WCET used to be about cycle accurate models but that's long been blown away by the complexity of real hardware. But there's quite a good bit of work on how to instrument real hardware and then pick up key subpaths measurements to assemble into a good bound. These guys have been doing this for a few years now and have a good tool:

https://www.rapitasystems.com/products/rapitime