There are various reasons why the IDE can be slow or unresponsive. The most frequent ones will be described here with hints how to improve the performance.
- Problems related to memory usage
If your Java process requires more memory than underlying OS can provide it can cause swapping of pages from/to disk. This results in significantly degraded performance. See the following topics how to address this problem: What are memory requirements of Netbeans IDE?, What is the usual memory usage of Netbeans IDE?, How do I set the JVM's heap size to be smaller? Contrariwise it is sometimes useful to increase amount of memory available to the application to allow it manage memory more effectively and this can reduce the time spent during garbage collection cycles.
- I/O operations slowing down the IDE
During some tasks the IDE is performing many I/O operations. With a slow device this can cause slowdown of the whole application. Typical examples are resources accessed on network drives or plugable drives like USB disk, storages using compression or encryption and anti-viruse applications guarding all accesses to files. Notebooks running on batteries have slower I/O too.
- Problems with graphics
Sometimes the performance of painting operations can be a bottleneck (or can even cause system instability). In such case it is useful to make sure that the latest available version of JVM is used as well as up-to-date drivers for the graphic devices. Java2D Graphics and Imaging page contains references to documents that can help here.
- Performance degrading over time
This can be a result of memory leaks and it often leads to an OutOfMemoryError later. It is discussed in What can I do if Netbeans IDE runs out of memory (OutOfMemoryError is thrown)? topic.
Too many add-ons and unstable plugins can affect behaviour of the whole IDE too.
Restoring of previously minimized application can take certain time because there is a lot of I/O (partly because the application can be paged out of memory, partly because of up-to-date check that is running in such a case).
Source: NetBeans User FAQ