Do speakers take time to warm up?

I understand your question in two parts:
a) do others hear the same and
b) if yes, why?
a) I hear the same, starting with and a long time with Maggies. The "simpler" ones had film caps, so no "real" cap forming was involved, and the degree of this speaker break-in is more and different from what I ever heard in caps & electronics. The Maggies sounded thin before "moving", my actual ribbon hybrids (dynamic bass to 400Hz) sound somewhat murky or "cloudy" when off for a certain time.
Leaving the system off for eg. 10 days necessitates several hours of gymnastics, the more the better.
b) as to why: I suspect the murkiness of dynamic drivers ( :-) improve a lot with gymnastics - but think about the damping, the mechanical hysteresis of the spider and surround, and even the membranes...
Interestingly Maggies, that have almost no damping, providing sort of a "clean spring" within the thin mylar foil, improve. Here it's probably a slight relaxation of that spring, probably affecting all planar speakers - ... including the newer plastic foil based Heil AMT drivers...?
Additionnaly to this there is probably a certain kind of static build up of magnetic remanence within generators, pole pieces,  traces of magnetic metals in contacts (nickel) and contact supports. This is probably kind of ho-hum demagnetized by music signals.
And moving a speaker (or other audio gear) from one place to another kind of takes time and energy to adjust.
In short: I hear the same and just don't listen too focused for several hours of reconnecting a system (instead: keep on playing music in a gapless mode... :-)

I experience the same, non scientific, thing. Speakers sound a bit better after a couple of songs are played. Even when the system has been on a while. 
The same with guitars. If a guitar hasn’t been played for a while, it takes a bit to open back up. 
Museums have musicians in to play the Stradivari violins. 
Instruments need to be played. 

I believe they do, my system, when first turned on is definitely bass shy. It takes a couple of songs to get everything up to speed. I have been experimenting with turning the amp, source and pre on and letting them warm up,but the results are the same. I'm thinking it's the speakers. I also have noticed this with other components and speakers.

Speaker coil resistance can easily change by 10% or more after warm up.  It will affect crossover design and I'm very surprised,  that people ignore it, discussing "crossover charging" instead.  Also, to people who cannot hear the difference - you don't have to.  Your hearing apparatus doesn't have to be the best.

My system is well broken-in.  Before playing music I always turn on the system for about 20 minutes to warm up the tubes.  Even so, the tube phono stage and tube preamp need to have musical signal passing through them for about 2 sides of an LP before they sound their best.  I will assume the speaker drivers warming up plays a role in this as well.  No matter what, the musical listening pleasure always increases around the third side of playing LPs.  And on Saturday night, after a couple glasses of wine and a couple of hits, it definitely sounds much, much better.
Enjoy the music!