Welcome to the forum!
I don't think your library is too big for MJ8. I used a bigger library without problems before I upgraded to MC9 a couple of years ago. However, the current Media Center is much better also with audio files in many ways. Upgrading to
MC11.0 may solve the problems you are experiencing, but that is not certain. The new database format is certainly better with big file libraries, but as you said the external HD may have some effect on this. What type of connection it uses? You should also mention your system specs. You can copy-paste your system info from the help menu.
If you like to try MC11 you need to uninstall MJ8 first. MJ8 and MC11 cannot coexist because the Windows registry settings would be mixed and the programs would not work properly. The database formats have changed in every MC version after MJ8, but I suppose the Library Restore function in MC11 can convert an MJ8 library backup file completely to the new database format. At least MC9 did that just fine when I upgraded.
Some time ago I tested several upgrade scenarios and posted my findings in this thread:
http://yabb.jriver.com/interact/index.php?topic=27914.0As a result of my testing I wrote these instructions for upgraders:
For the cleanest possible result I would recommend taking the following steps when upgrading from any previous MC version or from MJ8:
1. Make a library backup(s) to a safe place outside of the program folders. Each library needs a separate backup.
2. Copy the cover art folder to the same safe place. Make a note about the original path.
3. Make notes about the program options you have customized. You need to reconfigure them after installing MC11.
4. Uninstall the old version completely using its uninstaller. Check the program folders and delete all possibly remaining JRiver folders.
5. Return the cover art folder to the original location if it was deleted from the MJ/MC data folder.
6. Install MC11 and restore the library(ies). Check the program options and adjust if needed. Everything should be OK now.
A remark: Replay Gain analysis must be redone after upgrading from MJ8 (the system changed in MC9).
The thread has other more or less useful information besides my instructions. You may want to check it out. Also, a forum search might find more info.