It probably dates back to the old ECU's when OBDII first took over (1995). The ECU's of the day weren't all that fast so delivering serial information on the OBDII port could potentially make the ECU process engine data a few cycles slower. Probably not by much, and probably not enough to make a significant difference. In modern ECU's not likely at all, they have extremely fast processors compared to 15 years ago, the data on the OBDII port is nothing to them (even though the data rate is now higher).
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