Are you using Proteus Auto or Proteus Manual ? 
If you are using Auto, sure it will be slower because Auto have to analyse each frame and adjust parameters.
Also, what is your output setting ? What encoder did you use ?
The benchmark only calculate the processing speed, which does not include encoding. You may try export to images onto a fast NVME drive, to see whether your encoder/hard drive slower the process speed.
Quote from Developer,
