First, never zero an SSD. SSDs are self correcting. By zeroing the SSD you may have marked every block on the unit as needing a reset. It will vary from SSD to SSD since the firmware and how they handle bad blocks differs from unit to unit. Have you reset the NVRAM and done an SMC reset?
http://support.apple.com/en-us/HT201295
http://support.apple.com/en-us/HT204063
Second, it's not uncommon for our best friends, the MDS process (Spotlight and Time Machine indexing) as well as IconServicesAgent to kick off and bottleneck a system for a few hours. It depends on the OS you're running. The SSD will be seen as a new drive because the UUID of the drive wasn't anything in the system before.
Third. Possibly a damaged cable. SATA cables are notoriously fragile. The only product that can check them properly in real time is Scannerz (
http://scsc-online.com/Scannerz.html). The problem with SMART utilities like DriveDX is that if there was an intermittent disconnect it will log those errors and never clear them, even if it was a one time event. You'll never really know what the status of the drive is.
My advice may sound crazy, but here it is:
1. Assuming the resets do nothing, go to system preferences and set the system to never sleep, and NOT to put the drive to sleep when possible.
2. Leave the system on and don't touch it for about an hour or two.
If the drives firmware needs to clear blocks it will do so. If MDS and/or IconServicesAgent need to do their thing, they will do so. If after an hour or two the system has miraculously come back to life then those (possibly a combination of all of them) were the likely causes.
3. If it's still slow and you have an external enclosure, you could put the SSD into that and see if it's still slow. If it's still slow the SSD may be damaged. If it's not slow, then it points most likely at a bad SATA cable. Keep in mind that an SSD in an external enclosure will be limited by the data rate of the port, so if you put it in a USB port, the most you'll get out of it will be USB data rates.
A final possibility may be a firmware conflict between the SSD and the system. Since Mountain Lion, Apple has had problems doing a decent job maintaining compatibility with a lot of peripherals. Most of it's been USB and Firewire related, but I wouldn't be shocked if a firmware conflict existed with your drive as well. Not likely, but possible.