If you really mean a "lag" (ie, a time delay), that is completely irrelevant to the 8051!

All the 8051 needs is a good, clean edge - which is exactly what a comparator gives you.

The fact that the edge may be a little late is of absolutely no concern whatsoever to the 8051's input pin!
(you may have to compensate for it in your software, if it's that important to you, but it makes no difference at all to the operation of the interrupt).