I don't see where it's doing much. These are the heat indexes, calculated from the OP. It would be useful to know the outside RH, because that normally goes up as temp goes down. But the point is, swamp coolers make it more humid, so you really do need to adjust everything to the associated heat index to get a true feel for their effectiveness.
88/33=86 outside temp 84, 2 degree differential
86/40=85 after 1 hour
79/52=80 after 6 hours. outside 79 - 1 degree differential, not much different than starting
77/51=77 outside 68, time to open all the windows and crank the exhaust fans.