IMac – Best version of OS X for a 2008 iMac

Although you have stated that you want the best read/write speeds, you should consider other factors as well.

1. Redundancy (-Time Capsule; +iMac)

• With the libraries on Time Capsule, where would the backups of the libraries be? If you have not thought of that, then you must consider it. Having the libraries as well as their backups sitting on a single device, the Time Capsule, is not a good idea (even if feasible).
• With the libraries on the iMac, you could dedicate the Time Capsule exclusively for backups and have better redundancy.

2. Availability (+Time Capsule; -iMac)

• With the libraries on the Time Capsule, you would have them available anytime since the Time Capsule would be on all the time.
• With the libraries on the iMac, wake on network access, which is sometimes not reliable and breaks (and sometimes requires a patch from Apple) would force you to use other methods to wake the iMac manually (search through the Apple Support Community forums for wake on LAN or wake on network issues that have happened even recently).

3. Speed (=Time Capsule; =iMac)

• I do not have any practical numbers, and do not understand why the Time Capsule shows up so slow over Gigabit ethernet on the thread you pointed, but it may be a matter of time and a firmware update from Apple to resolve it.
• Although the native speed over SATA looks nice when the libraries are on the iMac, do you really need such speeds for music, photos and videos? I think not, even if you're planning for a future when 8K UHD videos become common (with Apple supporting it) and you plan to have large video files. Even 4K UHD (and the currently common Full HD 1080p) can be easily streamed at the lower end of the speeds you have noted.

4. Cost

• You would have to work this out yourself and see what you can afford and what makes sense.

Winner, in my opinion: Libraries on the Time Capsule with SSD for the iMac, plus another 2TB external drive connected for backups of the libraries on the Time Capsule. Back up the Time Capsule (entirely or just the libraries) from the iMac using a tool like Carbon Copy Cloner, SuperDuper! or Chronosync or using the Archive function from Airport Utility and an Automator script to make it periodic.

According to your comment, it looks as if you could do better with more RAM. If it was just launching slowly, then I would say the SSD, but since its switching apps also, I think more RAM would be nice. Unless your iMac ran a lot better under Mountain Lion or below, I really wouldn't downgrade.

BTW: Don't disable the user accounts. Just log off when you are not using that user account.

Since you have a spare SSD, you could also just connect it externally using an inexpensive USB to SATA adapter and install new OS on it and run some A/B tests. You won't see the full speed of an internal placement - especially if you use USB 2.0, but it's often enough to know rule out RAM or OS as a cause of slowness.