What you want IINA to do:
When displaying the subtitles found (using Opensubtitles.com, etc.) show also the FPS of the subs if available:

What IINA does currently:
It shows the subs list without this information. In particular it currently only shows:

• Language
• Download number
• Stars
• Upload date

Why you think this should be added:
This would benefit the choice of the sub since it depends on the video source FPS. It is quite common that multiple subtitles are found but I cannot choose the correct one and therefore find myself going to the site for a double check (which makes the in-player download feature almost useless).