To improve the efectiveness of openfire, you only need to increase the size of your JVM´s size memory.

I only found a option to modify the cache settings, adding a system property for each cache option. But, what is the default size for each one?, I don´t know and modify manualy have many risk.This is a wrong way.

The best way is to increment the JVM´s size and automatically the cache size increase.  I recommend you that the JVM'size would be at the relation of 1/2 of your physical memory, but if you have a good memory size maybe at 3/4, in dedicated servers. For example, if you have 2GB of physical memory, you could assign 1280Mb (1024 + 256) to JVM.

How do it?.  Find in this forums about the file openfired.vmoptions

Remember to use numbers base 2.

128

256

512

1024

etc...