internal SSD and HDD together speed
The DVD drive is connected via SATA rev. 2.0 / 3Gbit/s. That means the SSD is gonna run at a max. of 300 MB/s approximately.

This link seems to indicate that there's at least a second drive bay (excluding the optical bay) inside the iMac, which runs at SATA rev. 3.0 / 6Gbit/s speed. It would be wiser to use either the second bay or upgrade your primary system SSD.

Since the above link mentions the need to upgrade the EFI firmware, one should be aware the latest firmware updates can be found at the Apple web site "About EFI and SMC firmware updates for Intel-based Mac computers".

If you want to add an internal SSD, you can take a look at the ifixit guides, they can be used as a very good indicator as to how difficult the procedure would be.

I would not recommend an external SSD. They are extremely expensive and don't really give you a plus. Thunderbolt SSDs are even more expensive because of their rarity. If I were you I'd consider the internal upgrade first.