Perhaps one of the biggest problems with Path of Exile for the longest time has been the performance issues in the game client. It's not enough that the game used to have desync problems, but it also wasn't the most well-optimized game in the world. While far from being a complete wreck, it still hurts whenever the game stutters when too much stuff comes up on screen, which is usually during a really tough fight.
It's a legitimate concern, especially for those playing in hardcore leagues wherein even a little lapse can result in heartbreak. Even if you do everything right—get the build right, have all the Path of Exile items you need, and be strong enough in level to handle all those enemies with your POE items, an episode of lag or stutter can render all that null and void, or at least incredibly annoying.
Network performance is something that has already been tackled for the most part with the elimination of desync, although more can still be done on that front. As for game client performance, big strides have finally been taken when multithreading was introduced by content update 2.3.0, letting the client use more than one CPU core.
It may sound ridiculous to have a game run only on a single core in a time when multi-core CPUs are commonplace, but it's fairly common for various online games—especially free-to-play ones—to be mostly single-core applications since they're simpler to develop that way and would have lower system requirements to accommodate as wide an array of machines as possible.
The problem here is that once the game starts to have more things going on at once, like visual effects and other CPU-intensive processes, then it can greatly impact performance as that single core works on overdrive to do all that work while the other cores can't jump in to help out. This means the game client is bottlenecking its own performance by not having multithreading support.
When just a single CPU core is inundated by a workload that's better suited for a machine with multiple CPU cores, not only is performance limited, but it also puts quite a strain on the CPU courtesy of excess heat. That would then make the CPU throttle, which slows it down to keep it from getting way too hot. It doesn't help that certain maps in the game tend to have a whole ton of visual effects and other things going on.
But in 2.3.3, it gets better as a command line option has been added to let players choose to turn on frame multithreading, which allows the client to use more CPU cores for gameplay. It's granted that most would think this feature should be permanent instead of just an option, but Grinding Gear Games will still have to test things out in order to make sure that there won't be game-crashing bugs associated with the functionality. They hope to have the feature completely implemented without hitches by content update 2.4.0.
The next step after this is to improve graphics performance, which is something that could definitely have been useful a long time ago. Grinding Gear Games has been working on D3D11 support, which wasn't immediately developed due to focus on lower-end machines. Once It's fully implemented, Path of Exile should be able to tap into the power of newer graphics cards that support DirectX 11.