Sonic news website Sonic Stadium recently had some hands-on time with the new Sonic Forces game. Among their observations was a noticeable 30fps cap on the Switch version of the game.
So indeed it seems like Sonic Forces will take a hit frame-rate wise and also will take a hit in terms of visuals. Perhaps Jason from Sonic Stadium put it best:
All of this is almost invisible in portable mode outside of the locked 30 fps. I’m still considering this version and the PC version as the ports I have preference over, with the Switch offering portability and the PC for it’s eventual modding. If the Switch is your only option, it’s still a solid port – just think of it as a PC game in medium to low settings vs. ultra-high settings for the PS4 version.