"Paul" wrote
If there is a bad or fouled plug, you will see a drop as soon as the
good mag is switched off.

Yes, and the effect will be dramatic. The engine will shake like a
wet dog (especially a 4-banger) and the RPM drop will be both
immediate (as you noted) and very large.

If however there is a timing error which can be caused by the points
wearing, or the magneto itself slipping position due to loose hold down
nuts---then the drop will be much slower taking several seconds to
stabilize.

Maybe. It depends on which way the timing is wrong (early or late)
and how badly. Even worse, though, is what happens when the timing is
wrong because it's late. The common procedure for dealing with a
fouled plug is leaning the mixture as much as the engine will tolerate
to 'burn the plugs clean.' This actually works sometimes.

However, the nature of the ignition event is such that the flame front
propagates fastest with a mixture leaned to peak. Normally, the
engine is well rich of peak at runup power with the mixture set at
full rich.

Imagine, for a moment, what happens if the points have started to wear
on one mag and it is firing late. Unless that happens to be the only
mag with the impulse coupling, starting will not be a problem. At
runup, switching off the 'late' mag will produce a slight RPM drop -
so that part of the check is nominal. However, switching the good mag
off will cause a large RPM drop - probably more than allowable - and
some slight roughness. The pilot will then say "Aha! Plug fouling!
Let's burn those plugs clean." and proceed to lean the mixture as much
as he can. He'll run it that way for a while, then advance the
mixture only slightly (if at all) since he doesn't want to foul the
plug again. He checks the mag drops again. Now the drop will be a
little more on the good mag, and maybe a lot less on the good mag
(since it's late firing is somewhat offset by the faster combustion
event). Now the engine might just pass the mag check, and the pilot
will figure all is well - the rest will burn off in flight.

By the time this process stops working, meaning that leaning the
mixture will no longer result in a an acceptably low RPM drop, the
magneto is firing several degrees late. Not really a big deal in
cruise (especially low power cruise) but if the good mag (or even one
plug on it) decides to go during takeoff, it could really ruin your
day.

Michael