I have a 5k iMac (Late 2015), with a spinning hard disc, running El Capitan.
I'm considering enabling FileVault, but I'm concerned if it will hurt responsiveness. Will it? I understand that it doesn't on a SSD, but this machine has a spinning hard disc.
It should be fine as any bottleneck would be the CPU power involved in encrypting/decrypting the data as it is being accessed.
The slower drive would actually require less CPU power than an SSD.
Once the initial encryption has finished you are very unlikely to notice any difference in day-to-day usage.
Your iMac's CPU also contains specific features that assist with encryption (AES).