Actually, when I wrote bandwidth I meant the data flow/rate on display/panel interface on the hardware level, not on the display port. I used bandwidth for simple lack of better word, could have used data rate.
The point is, running 4K display (3840*2160) at 90 Hz requires 812 MHz pixel clock. Running 2560*1440 (Pimax 5K display and Pimax 8K resolution) at 90 Hz requires 368 MHz pixel clock. So the data rate required by 4K display is more than twice compared to non-upscaling solution.
The panels Pimax is using for 8K seems to be not rated at these operating speeds, so they face problems. Using lower res panel with significantly lower data rate may actually give better chances to make it operational at 90 Hz.