Assume you have quite long method with around 200 lines of very time sensitive code. Is it possible that extracting some parts of code to separate methods will slow down execution?
Most probably, you'll get a speedup. The problem is that optimizing a 200 lines beast is hard. Actually, Hotspot gives it up when the method is too long. Once I achieved a speedup factor of 2 by simply splitting a long method.
Short methods are fine, and they'll be inlined as needed. So the method call overhead gets minimized. By inlining, Hotspot may re-create your original method (improbable due to its excessive length) or create multiple methods, where some of them may contain code not present in the original method.
The answer is "yes, it may get slower." The problem is that the chosen inlining may be suboptimal. However, it's very improbable, and I'd expect a speedup instead.