On less that stellar mags, the solid impact of the buffer to the back of the receiver extension adds the mag on bumping up the top round to the feed lips. If a heavery buffer is added, and the gas system is funtioning on the light side, then the buffer never gets fully slamed off the back of the receiver extension.
This means that yes the bolt is opening up slower, but without the dead blow effect of the buffer off the back of the receiver extension, the carrier is coming back too soon/faster than normal (ammo not fully raised to the feed lips). On the rifle in question, it could be that that is has some gas leaks, the chamber is on the rougher side, or even that the carrier is binding up on the hammer (disconnector wedging on top of the trigger, not allowing the hammer to be lowered enough without binding).