It would seem that using the ASCII charset for most fields and then specify utf8 only for the fields that need it would reduce the amount of I/O the database must perform by 100%.
Anyone know if this is true?
Update: The above was not really my question. I should have said: use Latin for the default character set and then only specify utf8mb4 only for the fields that need it. The thinking being that: using 1 byte vs 2 bytes should improve I/O by 100%. Sorry for the confusion.