If you want the top speeds of Forza Horizon 5 accessed via a mobile device, you aren't using a PSP. You are likely using a smartphone via . In this setup, the "PSP" is replaced by a Backbone or Razer Kishi controller attached to your iPhone/Android. Visually, on a small screen, the streamed version of FH5 looks shockingly similar to what a PSP successor might have played like.
The "Forza Horizon 5 PSP" doesn't exist. But the era of is already here. You just have to look past the 2004 hardware and into the cloud. forza horizon 5 psp top