I'm trying to understand why this is happening: the problem seems to happen ONLY in Full screen mode, so it's surely some DirectX issue with text in Full screen mode. Unfortunately, this readout must be updated at every frame, otherwise it's not very useful. The fps doesn't change a bit in Windowed mode.
For the time being, either use Windowed mode, or reduce the "Verbosity" slider in the "Simulation" page of the "GSX - Settings" menu to one notch, so the readout won't be displayed.