Yep, because in V3 demo with GTX 1070 we can see they render at 4096x2657 /eye, that is much higher than 2560x1440 input per eye, so would compromise performance(FPS) on any GPU intensive VR game. It is basically like rendering with Vive but set SS to 2.0 in SteamVR, and it is known that only GTX 1080Ti can handle such settings(with decent fps), hope Pimax can optimize this, but there is inherited need for higher render res(than native) to get 1:1 pixel mapping in center of the lens due to higher FOV. So number 4096 may be lowest optimal, 2657 probably can be lower. Wild guessing a bit .
More wild guessing, why use 2657, that is 1.85x1440, is to compensate for upscaling text. I think when upscaling, it is more preferable to reduce horizontal res, if I remember correctly PS4 pro uses checkerboard rendering at res 1920x2160 so keep vertical res. Why is vertical res so important? Text usually has a lot of strait lines, and upscaling will cause text not to look as sharp(less readable). Pimax upscales vertical from 1440 to 2160, Linus already noted that for this case, if you want to read text, 5K could actually be more optimal(no upscaling).