UTF-8 and Latin1 are typical encoding for MySQL query.
When inserting BLOB:

# Decode binary data
x = data.decode('ascii', 'surrogateescape')
# %-format query
psql = sql % (escape(x),)  # sql is unicode
# Encode sql to connection encoding (latin1 or utf8)
query_bytes = psql.encode('utf-8', 'surrogateescape')

So decoder speedup is required only for ascii, but encoder speedup is required for utf-8 and latin1.

I'll consider other ways including creating speedup module and register it on PyPI.