From Nathan Hyatt@1:2320/100 to Joseph Larsen on Sat Apr 16 19:51:02 2016
{$F+,I+,L+,R-,S-,V-,Q-}
$F+ - provided by FPC for compatibility with Turbo Pascal. Has no effect on
32 and 64-bit compilations
$I+ - still used, see freepascal.org/docs-html/prog/progsu38.html
$L+ - ignored by FPC
$R- - disable range checking, still used
$S- - disable stack checking, still used
$V- - disable strict string compatibility checking, still used
$Q- - disable overflow checking, still used
See freepascal.org/docs-html/prog/progch1.html for all available compiler directives.
Seems to me most of these were originally selected to optimize for speed back in the day.