The overhead of, for example, invoking a method via reflection is measured in single-digit nanoseconds. Still not as fast as direct invocation, but unlikely to be a bottleneck unless you perform 10-of-millions of them per second.
Although I can't tell you why, it's been my experience that firing a simple method via reflection in both .NET and the JVM is roughly 2x the time of direct calls (or of calls via the 'dynamic' keyword in C#).
