In recent years, real-time physically-based solutions for sound synthesis have become a common practice both in academia and commercial applications, driven jointly by the increase in computing power and advances in modelling and simulation techniques. However, designing and implementing such instruments and the means for expressive interaction with them still poses several challenges, factors to be balanced by the instrument creator depending on his or her needs. This paper discusses these concerns through the prism of mass-interaction physical modelling. First, a short review on the topics of computation and expressive control of real-time physical models is proposed, followed by the introduction of the physical proxy, a new mass-interaction modelling element yielding new possibilities for expressive interaction via dynamically interpolated topological connections. Three case-studies of physically-based virtual musical instruments are then presented and discussed, particularly in regards to computational aspects, with benchmarks of various possible implementations. Finally, we offer some insight on the balance that one must consider between model and control complexity, and between software genericity and performance, when building physical models for real-time sound synthesis.