PSA:
Set PS5 video output to 60HZ!!!
Not a bullet point in the OP.
I'm sure it's been said here already, but it's a pretty crucial bit of information for not having a laggy shitfest on/in your hands!
Reddit/Twitter/etc. is currently littered with Luddites that haven't figured this out/actually read the set up guide.
Smart shoppers will keep a close eye on Facebook marketplace and the like after Christmas due to the potential droves of dummies that don't figure this out and ditch their gifted Portals on the cheap;}
Again, we have to see the hard evidence/receipts, just 'cause people on reddit said so (God knows the amount of BS goes on in there) doesn't necessarily make it so.
The 120Hz thing is something that happens between the PS5 and the TV - let's call it a "handshake" - you give the "OK" from the PS5 to display at double the refresh rate IF the other device connected (your 4K, 120Hz VRR TV) can support it, if it does, then and only then does the TV start displaying at 120Hz (more bandwith).
When you take the TV out of the equation...nothing happens, the PS5 doesn't suddenly start outputting at double the framerate/refresh rate to something that doesn't support it (even via mirroring/casting which is what the portal basically does) at double the bandwidth since you PS5 can operate with the TV turned off.
What is happening most probably, is that people have enabled the various 120Hz game modes via in-game options with some of them running at unlocked framerates and while the various framerate fluctuations aren't as visible on the TV due to VRR, it's something that is indeed visible/palpable on the portal's screen.
For the time being, unless someone from a reputable technical channel comes forward and mentions the exact opposite, I'll have to chalk this up to Placebo.
Again, I'm one of the "lucky ones" that is using RP on his tablet (Tab S9+) with absolutely 0 problems
and i do have my TV set up to display at 120Hz (LG C2), then again, my PS5 is hardwired to a non-shitty ISP provided router (TP Link archer Ax5), I've set static IPs to everything, I've given my PS5 bandwidth priority through the QoS menu from within my router , I have a pretty steady 1Gb connection and my tab's wi-fi antenna is also good.
Unfortunately streaming is what it is - there's just too many variables involved causing each user to have a completely different experience from one another.