11,000 photos is not too many- I have 50,000 - others have more
Not sure where you heard that multiple libraries is the solution but it only is to the extent that creating a new library usually will eliminate the issue that caused the slowness - the better solution is to fix the slowness - it is like buying a new car because your brakes are wornout
Slowness can be preference settings or can indicate corruption that a rebuild will address
To create multiple libraries depress teh option key and launch iPhoto and use the create library command - to switch between tehm option launch iPhoto and use teh select library command iPhoto Library Manager - http://www.fatcatsoftware.com/iplm/ - is very useful in managing multiple libraries
How much RAM does your iMac have and what other App's or Processes are running at the same time when your working in iPhoto?
I ask, because I have two Mac's that both have over 12,500 photos in there iPhoto Libraries. The older one with less RAM handles iPhoto nearly as well as the newer one with 8GB of RAM, as long as there are not to many other App's or Processes running at the same time.