> compilation itself (including ocamlopt.opt) took some time, which
> didn't pay.

There must be an adapative break even point though. e.g if you're
n/m pixils done in s seconds, and the average cost of compilation
is c times the simple rendering speed per line and the average speedup
is q, then subtracting the pixils already done, work out whether

	(m-n) * p < c*lines*p' + (m-n)*p*q
	where
		p = (m-n)/s 		rendering speed
		p' =			simple rendering speed (speed of
					a delay loop or uncomplicated pixel etc)
					

		
And if so, compile.