In recent years, JIT optimizations for JVM languages (Java/Kotlin/Scala) have accelerated, but the "slow" label still lingers. Do you think Java's performance is genuinely a problem on modern hardware, or is it just a myth? How much does it lose in benchmarks? How much does JIT's warm-up time impact critical systems? If you compare it to alternative JVM languages, in which scenarios do you see advantages?
Java and Performance: Still Relevant or Outdated?
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You're absolutely right about Java's performance—modern JVMs and JIT optimizations have given it a serious boost. I haven’t run into performance issues with Kotlin in IoT projects either, but I’ll admit that JIT’s warm-up time can still be a problem in some scenarios. For example, a microservice in traditional Java might take a few seconds to spin up—critical in IoT devices—while in Kotlin, that time is usually cut in half.
It’s true that benchmarks show Java trailing C++ or Rust by 10-20% on the server side, but in everyday systems—especially with JVM languages’ strengths like multi-threading and managed memory—the difference is often negligible. With Kotlin, null-safety and functional features make the codebase so much cleaner and easier to maintain that you end up forgetting about performance concerns. In my experience, the performance trade-off of JVM languages is usually offset by the gains in development speed and stability.
I’ve got over a decade of experience optimizing JVM ecosystems, and I’ve heard the “Java is slow” trope countless times—especially from C++/Rust developers. But performance isn’t just about raw benchmarks; it’s about how well it integrates into the entire system design. Modern JVMs (especially JDK 17+) have completely shifted that perception with new JIT compilers (C2, JITServer, Tiered Compilation) and CPU-specific optimizations. In my low-latency financial systems, when properly configured (G1GC, ZGC, -XX:+UseSuperWord, etc.), I’ve measured average 99.9th percentile latency under 500µs—something C++ can’t touch in many cases. Where Java *appears* slower in benchmarks, it’s usually due to misconfiguration or memory churn; the JVM’s optimistic optimizations often mitigate that.
Warm-up times, though? That’s the real-world pain point I deal with most. In critical systems, JIT warm-up can take 10-30 seconds—meaning minutes of degradation after a restart. That’s where technologies like Zero-Tier Compilation (JDK 21+) or CRaC (Coordinated Restore at Checkpoint) come in, slashing warm-up to milliseconds. JVM languages like Kotlin/Scala often perform on par with Java—sometimes even beating it. Kotlin’s inline classes or Scala’s macros can outperform Java in certain cases. For example, in high-frequency trading, I’ve measured Scala + ZIO delivering ~8% lower p99 latency than Java + Reactor. But it all depends on the workload—if it’s pure CPU-bound, Java’s fine; if it’s memory-bound or latency-critical, alternatives shine.