

I gave it some thought, I think that you are getting slowdowns because of some kind of a bug and not due to slow speed of the GPU.
I have actually daily-driven a MacBook Pro 15-inch 2009 with a GeForce 9600M GT and even at 279 Mhz core, it was usable on Manjaro KDE, animations were a bit laggy, but nothing compared to what you are describing.
I still remember trying kernel 6.7 or 6.8 and immediately seeing MUCH worse performance with constant lags. I have only consistently used kernels 6.1, 6.6 and 6.12 on Manjaro on that machine, all of them with decent experience. I would try some other kernel if that’s possible, but considering that you have tried 6.12 and 6.16 at this point, I am not too hopeful.
Depends, if it is used as a dumbphone (calls, SMS, being in sleep most of the time), it is okayish. However, going outside dumphone sphere, it becomes cumbersome to use.