MacOS – Is Running MacOS Off a Flash Drive a Good Idea?

On the immediate downsides - You will need special tools and be handling some delicate parts. Also, there is concern that the temperature sensing since the 2009 model iMac causes the fans to run faster with non-apple drives.

If you really want to get dust between the LCD and the front glass on a brand new mac and take the chance of ripping a tiny ribbon cable - just do some research at ifixit.com or talk with a local tech that works on new iMacs a lot. Do also research fan control and know if you are not planning on an Apple specification drive to install.

My guess is Apple SSD are expensive because the parts are cherry picked to be the best of a lot (more so than the SSD consumers buy) and perhaps loaded with custom firmware and generally have temp sensors specific to the mac. I could be totally wrong, but even if you feel the drives are overpriced, Apple installing any SSD is better than a consumer due to the labor for for installing them is almost nil as they are installed at the most economical time - at assembly.

If you can't afford it now - why not expand to an external SSD over firewire or ThunderBolt later. They are totally silent and small. It just seems the risks of DIY on a new mac make it better for most people to pay a trained tech to do the install.

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.