This article is from the comp.sys.acorn FAQ, by Paul Vigay with numerous contributions by others.
On RISC OS 3.5 and 3.6 machines there were seen to be serious delays in the desktop when running applications with large wimpslots, mostly indicated by the lack of reponsiveness. The problem was identified as being that of having to page in the entire application which, with a 4k page size was quite slow. In RISC OS 3.7 this situation was improved by changing the way in which the application is paged in.
RISC OS 4 can support a new method of paging tasks in - Lazy Task Swapping. This can also be called 'demand paging', because instead of paging the entire space in, individual pages are only given 'on demand'. This means that in a large application only sections of the application space may be physically present although the application itself will be unaware of this.
There is a problem, however, with some versions of the StrongARM processor which will (under certain circumstances) cause Lazy Task Swapping to fail. StrongARMs prior to revision T exhibit this failing and Lazy Task Swapping is consequently be disabled. You may also want to disable Lazy Task swapping if you encounter problems when using DMA.