Of course you have to understand in my context I've really come in close intimate contact with Java twice. The first time about 14-15 years ago when the JVM was pretty doggish. I
think it was Java 1.4 or 1.5. I don't think Eclipse was even out then, and the first version I remember seeing was so dog slow at the time compared to Visual C++ that I remember just writing off Java entirely.
To my surprise it stuck around in Enterprise spaces and I ended up running a team that ported a pretty large-ish Perl project (about 20k lines of Perl) into a piece of Java middleware. I didn't do any of the coding, but I knew the Perl bits inside and out so I knew what kind of testing and performance to expect and getting the Java port up to 1/3rd to 1/2 the performance of the original Perl was a pretty big challenge at the time. This was about...8 year ago. But to the credit of the team and I guess the tooling, they were able to get the port done remarkably quick.
So I've been pretty surprised to see that Java code can get fast, really fast. I've also managed to bang out some pretty slow code if I wasn't paying attention. For my latest touch into it, I ended up spending a week just benchmarking various collections to see how they actually worked under the kind of real-world uses I'd be putting them through (vs. what their big-O sheets claimed) and came away with a set of practices for me to get going with.
I've also had to come to terms with benching code a bunch of times to give the JIT compiler or whatever runs under the hood in the JVM some time to optimize, the first few runs are invariably much slower than run 1000.