In an unrelated thread, someone posted this quote from the Mithril documentation on the differences from React, which sums it up pretty well: https://news.ycombinator.com/item?id=22777320

Java and React are fast by bringing a lot of sophisticated (heavy!) machinery. Nim and Mithril are fast by being small and simple.

For example, the JIT makes Java fast – eventually. But initially it's interpreted and slow, with the additional overhead of bringing up the JIT compilation machinery in the background. AOT compiled code reaches its normal speed from the start. So Java programs take a while to get fast, which makes them feel heavy.

Startup is slow. Java 11 is slower than Java 8 which is slower than Java 6. Class-data sharing can make it faster – sometimes. You still have to load all that data, so when it's not cached in RAM and you have a slow disk it's still slow. A smaller program is always fast to load. This makes Java feel heavy.

When it comes to memory, Java does clever optimizations like escape analysis at runtime so that the programmer doesn't have to bother with deciding between allocating on the heap or the stack. This can also make it fast in certain scenarios, after warm-up, but a language with explicit value types can be made fast from the start. (Which is why Project Valhalla is coming.)

Startup is always slow (I am really sensitive to latency, and I prefer to have some immediate response from software). You either need to tell people to install JRE beforehand (which nobody will do), or pack it with your distribution, adding 100 MB or so even for simpler programs. JVM-based software is fine on the server side, but I'd prefer something else on the client side.