If both vertical and horizontal dimensions are to shrink, with scaling, yes, that might require more processing. However, if some areas are just not included, particularly if only the horizontal FOV is changed, I think the onboard chip may be able to handle that for two reasons.
First, a lot of headsets have different aspect ratios, and it makes sense that the panel driving circuitry would be generic enough to accommodate a variety of such modes without wasting DisplayPort bandwidth. The relevant chip could well be an industry standard DisplayPort to LVDS deserializer chip, that outputs a stream of raw bits which are ultimately fed to the active matrix panel perhaps largely unchanged from there.
Second, in my experience, the Pimax 5k+ seems to reduce FOV at very high refresh rates, or at least sometimes seems to be more reliable at high refresh rates if FOV is reduced. Also, PiTool often seems to change my FOV without asking after I try to change the refresh rate.
Overall, I think something like a 114Hz native mode at Small/Potato FOV might be possible IF the GPU DisplayPort circuitry can support suitable modes.
As it is, Pimax talk about ‘working with NVIDIA’, and the need for new drivers, suggests to me that the Pimax Vision 8kX might just have needed an entirely new non-standard DisplayPort mode added to get to 90Hz native.