The "lazy" encoding functionality allows NX to defer the most bandwidth consuming operations to give precedence to interactivity. By keeping track of modifications of windows and pixmaps, the NX agent can reorder the screen updates or discard intermediate drawing operations in order to guarantee a high level of responsiveness even with a graphically rich desktop over slow links.
There are two levels of lazy encoding. The first level drops the drawing operations on the offscreen, and keeps only the onscreen data updated to spare all these pixmaps that don't need to be immediately visualized. This level is applied by default when the link type is WAN.
The second level of lazy encoding makes the NX agent able to handle both the onscreen and the offscreen operations, by taking full control of the X clients and to decide if and when it is time to update the windows content. The screen is updated during the intervals of user activity and when the network is not congested. This level is applied by default to MODEM, ISDN and ADSL links, giving sensitive improvements to the NX session responsiveness that can be perceived for example when reproducing a video or playing a flash animation.
Even if the default level of lazy encoding is chosen to offer the best responsiveness according to the link type, it will be always possible to override this setting via NX Client GUI -> Advanced tab, by enabling or disabling deferred images updates, or in the NX session by pressing the Ctrl + Alt + E key sequence.