Max 10 - Difference in performance between low-speed and high-speed I/O pins

I don't think that the old post is inaccurate. The quoted 310 versus 310 Mbps numbers are correctly quoted from the datasheet.

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one can only assume that the low-speed would be less than that. IMO 150 Mbps/MHz is a reasonable limit for low-speed if high-speed is 300.

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You probably miss the important point. It's so that so called "low-speed" pins don't support certain IO standards at all, e.g. no true LVDS. The speed difference for the supported IO standards is almost negligible.