There's a number of rumors, but they kind of run the gamut.
Word is Square has devkits running FFVIIR, so assuming they have it running decently, that's PS4 level. The dev preview at Gamescom supposedly had Nintendo showing a souped up version of Zelda BoTW, which doesn't sound that impressive. But on the other hand also the UE5 Matrix demo, which might indicate something roughly in the ballpark of Series S. Tho I don't believe in Series S level. That kind of hardware in a handheld factor would be too expensive, and Nintendo does not sell systems at a loss. Also not feasible at a 15w tdp, and anything much more would have terrible battery life (see Steam Deck and such).
If I had to guess, I would put it around the PS4 (regular, not Pro). It will probably have a better cpu, tho that's not hard as the PS4 cpu was pretty garbo. And an overall weaker gpu in raw horsepower, but with more modern tech like DLSS or equivalents to compensate.