The reason for slow play on the pro golf tours is simple: the pros are too good. When the players are this good, the main differentiator between winning and missing the cut can be as simple as putting, chipping, and distance-control in windy conditions or bad weather.
Best example: when two pros in the same group have essentially the same putt except one is slightly closer to the hole than the other, the second-to-putt is almost a lock to make the putt unless it is extremely long or the breaks are extremely severe. When the players get to be this good, there is an extraordinary premium on informational input, which means the problem is only going to get worse.
Putting the players on a strictly-enforced time clock would solve the problem, but that would fundamentally alter the nature of the game. Hasn’t worked in baseball, where the slow-pitch rule is almost never enforced, and wouldn’t work in pro golf. Probably would work fine for ordinary golfers – just send the marshals out “heavy” (as they say in the Bourne movies).