You're missing the point. Using a character class - especially a negated one - creates additional work for the regular expression engine. The simpler the match, the quicker the function. A character class is slower than a wildcard match; The . wildcard is particularly efficient, since the regular expression engine doesn't have to care so much about what it matches.
The byte-size of your regular expression has very little to do with its efficiency.
Strictly speaking, if you wanted to improve the execution speed when dealing with large amounts of text, you'd use something along the lines of
alias a4 return $iif([ isin $1-,$regsubex($1,/[.+?]/g,),$1-)
Since an if/find operation is vastly quicker in execution than a regular expression.
Hope my explanation helps.