Frankly, I'm surprised that SSD works at all in the optical drive bay.
Do you see that diagonal ribbon cable running across the top of the SSD? That's the cable for your WiFi and Bluetooth module. It passes directly over the SATA connector of the SSD. The 15" and 17" models from 2011 in particular have an electromagnetic interference problem with SATA III devices because of the way those connectors perfectly line up like that.
This is a "design flaw", but I use that term in quotes because it works exactly the way Apple intended. They never planned or supported anything other than an optical drive there. Yes, the port is SATA III, but their optical drives are only SATA I and those work fine (SATA III was a brand new thing in 2011).
The SSD might work faster if you swap positions with the HDD. It's entirely possible that you've been experiencing errors the whole time and just never noticed because the SSD's error correction is silently coping with them. But constant error correction means slower performance. Usually people report strange behavior and weird corruption issues with SATA III drives in the optibay, so if yours has been working fine then maybe you just got lucky? Maybe the caddy you're using is high-quality? Or maybe your laptop is from the early 2011 batch that only had a SATA II connector in the optical bay.
Anyway, from that picture it looks like you've still got the factory HDD installed in the HDD bay, so there's a good chance it's also a SATA II drive. There are no issues with SATA II drives in the optibay; only SATA III, so swapping the positions of the two drives may make things better for you.