In my experience, if you write Java like you'd write to, you'd probably get similar performance. Maybe AOT performs a bit better, maybe it doesn't because it disables live optimizations.
Java doesn't lend itself well to writing Go style code, though. You quickly end up writing very "Java" Java code. That has some advantages (code deduplication, less boilerplate) but also downsides (allocation overhead, performance impacts).
In terms of Java vs C#, I don't think you'll notice too many new issues. If you opt into using heavy frameworks like Spring Boot to solve annoying problems for you, you'll see RAM usage increase massively, but it'll also take care of a lot of annoying grunt work for you. Startup times overall are pretty similar. The biggest downside in my experience is the lack of nullable types (@Nullable annotations are a poor substitute) and some syntax improvements that are new to Java but have been part of stable C# for many years.
As long as you stick with small libraries and avoid writing too much ObjectInjectorFactoryProducerResolver style code, performance of the JVM is fine.
All of that goes as long as you're able to use modern toolkits. If you're stuck in the world of Enterprise Java with Oracle Java 8, you'll notice old Java's shortcomings a lot more.