Author here. Both the JVM and CLR do some pretty aggressive optimizations even for normal interface calls. In the Java section I also show inline caching, a standard optimization approach. The CLR calls its approach to caching “virtual stub dispatch”. Speculative optimizations and inlining would be more advanced optimizations that I did not cover.

But what about the worst case – the method lookup when the method is not in the cache? There, the CLR introduces an additional level of indirection for calls through interface types, which has benefits for memory usage and possibly dispatch time. But since this is the worst case, the complexity doesn't matter that much as long as the important cases (which can be cached) work well.