Opening the exhaust early subtracts from the duty cycle (less energy turned to crank rotation, more lost in the pipe) in favor of reduced pumping loss.
Closing it later (during OL) leaves more exhaust gas in the chamber, which both dilutes and heats the new charge. Typically cylinder pressure is higher than we want (I want zero!), so the exhaust forces the new charge backward until RPM is higher (with high vacuum this is even worse). This is one reason why "split overlap" (equal duration IO BTDC and EX ATDC) is nothing like equal events in terms of pressure balance.
Again, it's a compromise like everything else.