"Nothing but C is fast [enough]" is an early optimisation and wrong for all the reasons that early optimisations are wrong. If your system has enough complexity that something other than C is desirable, then there will be parts of the system that must be "fast enough" and parts with lighter constraints. If writing your code, for example, in Python will get the project finished faster, with fewer bugs, then you can follow up with some C or assembly code to speed up the time-critical parts.
Even if it turns out that the entire code must be written in C or assembly to meet the performance requirements, prototyping in a language like Python can have real benefits. You can take your working Python prototype and gradually replace parts with C code until you reach the necessary performance.
So, use the tools that let you get the development work done most correctly and most quickly, then use real data to determine where you need to optimize. It could be that C is the most appropriate tool to start with sometimes, but certainly not always, even in embedded systems.
