Java isn't exactly fast either. It's definitely faster than BYOND, but when it comes to games I personally think C++ is best choice.
To clear things up here, I don't think either one is better than the other overall; that's a choice solely for a programmer - understanding the technical differences between both of the languages will immensely help you instead of just saying "C++ is better than Java when it comes to game development".
I've played games made with Flash (a lot of Kongregate games), Unity (a few on Kongregate) and Java (Runescape, Spiral Knights, Minecraft, Drakensang) and the only constant thing about them was lag.
As far as I know, rendering images exceptionally fast(which is what I'm going to guess you're expecting) is a known problem in Flash.
As for Java... Minecraft has slow chunk generation, lag spikes when I turn the camera, when I tried to host multiplayer it was horrible as well, only 1-2 players were able to play lag-free. Loading texture pack with high resolution textures takes like a minute. 4096x4096 texture should be about 64 MB in size; data bandwidth is several GB/s, so it should be within few miliseconds to create such texture, no idea why Minecraft takes so long. Runescape has generic low FPS through whole game, considering it's an MMO it could be fine, but other MMOs don't suffer from such problem, so I'll just blame it on Java, unless you think Jagex doesn't have good programmers. Spiral Knights had random 1 second spikes in some levels, as far as I asked people everyone had it, 'nuff said.
I have never experienced these problems and I usually play Minecraft on my AMD Athlon Neo Processor w/ 2GB RAM.
Java isn't exactly fast either. It's definitely faster than BYOND, but when it comes to games I personally think C++ is best choice.
I've played games made with Flash (a lot of Kongregate games), Unity (a few on Kongregate) and Java (Runescape, Spiral Knights, Minecraft, Drakensang) and the only constant thing about them was lag.
Some Flash games had laggy input while graphics clearly aren't lagging (Epic War Saga, Shop Empire 2), while Ninja Warz had no input lag but there were extreme graphical bugs (half-rendered images, etc), whole screen gets messed up because Flash just can't draw fast enough.
For Unity I can remember only Accelerator, maybe it was resource creation or GC, but there were some random lags, considering game's style it was really bad.
As for Java... Minecraft has slow chunk generation, lag spikes when I turn the camera, when I tried to host multiplayer it was horrible as well, only 1-2 players were able to play lag-free. Loading texture pack with high resolution textures takes like a minute. 4096x4096 texture should be about 64 MB in size; data bandwidth is several GB/s, so it should be within few miliseconds to create such texture, no idea why Minecraft takes so long. Runescape has generic low FPS through whole game, considering it's an MMO it could be fine, but other MMOs don't suffer from such problem, so I'll just blame it on Java, unless you think Jagex doesn't have good programmers. Spiral Knights had random 1 second spikes in some levels, as far as I asked people everyone had it, 'nuff said.
Edit:
Before you say I need better PC or something like that. I cna play Assassin's Creed on highest settings, Crysis 2 on medium (debug/beta version of Crysis 2 reported rendering 700,000 triangles per frame), there's no way this PC can't handle drawing Runescape with it's 500 triangle models and near-to-none shading.