Hi Arturo,
The M3 pause is no problem when used with a spindle motor but, as you have found, it is undesirable with lasers. Others have been here before and have had success by using the undocumented M10/M11 combination, which is much faster but requires careful programing to get it just right. This link refers :-
http://www.machsupport.com/forum/index.php/topic,6629.msg43375.html#msg43375I have found that the M7/M9 combination work for me (this is almost 10xfaster than M3/M5) and can be easily configured using a GCode editor to modify the standard GCode program.
Tweakie.