In response to customer feedback with regards to native image generation (NGen) taking too long (and in some cases leading the user to believe that the installation process had hanged), we separated the NGen process into pre and post-installation NGen processing since Exchange 2007 SP2 Rollups. During pre-install only non-Exchange binaries will be processed (the computer might have had updates installed for non-Exchange files that Exchange depends on, such as .Net framework hotfixes). The idea behind this is to keep Exchange services running while doing non-Exchange related work; only making Exchange unavailable when absolutely necessary. The post-install NGen of Exchange files can still take some time but the time will generally be much less with this pre-install feature.
During post-install NGen, as implied above, only Exchange files are processed. We have a progress bar for post- install NGen to show when (approximately) it will complete. We would like to display a similar progress bar during the pre-install NGen as well, but since we do not know how many files will be processed, we cannot estimate completion of the task.