My example of library speed:

• 8 runtimes altogether (1 Poser main, 7 additional ones on a second internal harddisk)
• Content about 21 GB spread over all 8 runtimes
• Option "Reopen Folders" is active
• None of of the runtimes are excluded from online virus scanning

Startup time time of Poser (after a PC reboot - so nothing is in the cache) including opening of the library and opening the folders from the last session: 20 seconds.
Navigating through the library (opening folders, changing the runtime etc.): under 1 to a max. of 2 seconds.

Conclusion:
For me the librarys speed is absolutely ok.
I feel that: - defragmentation gives speed

• not using an external hdd (especially USB ones) gives a lot of speed
• using a fast internal hdd gives speed
• having a few smaller runtimes instead of one big runtime seems to help (but I never tested with only one big runtime)
• on slow PCs with slow hdd excluding the runtimes and the working dir (where your scene files are) from online virus scanning might get some extra speed