One of my neighbors was a highly regarded formula car mechanic. He worked for several elite racing teams, for drivers whose names I think you would probably recognize. This guy was expert on Lotus engines, and undoubtedly, he could tell you that as great as those machines were, if the timing was off, they would perform quite poorly. In the old days, engines had vacuum advance hoses that, when the engine sped up, advanced the spark in the cylinder, because when the speed of the engine changes, the timing of the spark has to change as well–the faster the engine is going, the sooner the spark has to arrive in the cylinder. Nowadays, computers do that task, but the point is the same: as speed changes, timing must adjust to it.
Now, how does this apply to the golf swing? Tempo is the overall pace, or speed, of the swing, like engine rpms. Timing is the sequencing of the positions within that time frame. If the club or body is in a subtly different position at a given time, in reference to the rest of the swing, bad timing is the result. The spark then arrives in the cylinder at the wrong time, so to speak. In a nutshell, even a Lotus of a golf swing doesn’t work as well as it should when the timing breaks down. Now, herein lies the difference between your swing and that Lotus engine: your swing does not have a functioning vacuum advance. In other works, if the tempo of your swing changes, the timing will not readily adapt, because in the time that it takes for your instincts to recognize that your tempo changed, the ball is gone–Elvis has left the building! This is why we need to spend time practicing our tempo–consistent tempo is mandatory for good timing.
My favorite aid is the Pro Tempo Tuner, music specifically geared for golf, accompanied by voice prompts. (tempotuner.com) You can use music, count (1,2,3, swing!) or anything else you can think of to promote rhythm, but the Tempo Tuner won’t inadvertently “speed up the metronome”, a plus.