Does a huge key length value for a mulibyte column affect the index performance?
The number is a worst-case estimate. Remember that EXPLAIN doesn't typically read the data, so it has no idea if your data contains only a short string of 1-byte characters on average, or if it contains a full 64 characters each 3-bytes per character.
The performance of the query is much more influenced during execution by the number of pages read, and that varies more directly with the actual data. Short string values will allow more index entries to fit per page, therefore fewer pages need to be read. This is something the EXPLAIN cannot predict.
I would recommend do NOT change your encoding to latin1 solely for the sake of making EXPLAIN output &quot;less confusing.&quot; This is not a good reason, and it's bound to limit you later if you want to support UTF-8 characters in that column.
In today's era, there's virtually no reason to use the latin1 character set. I recommend setting all strings to utf8mb4 as a default.
If the column has something like strings of hex digits for a uuid or a hash value, that's different. You could set the character set to ascii. But that's for a reason of the data, not for the convenience of someone reading the EXPLAIN.