As uncle Albert (Einstein) used to say:
"The Lord God is Subtle, but Malicious he is not" (May, 1921)
From class demonstration & lab #1: When a string/slinky is stretched
the waves move faster. Also, when the string/slinky is more massive the
waves take longer to come back, so the waves go slower. The combination
of the two will give
Either a slower wave OR a faster wave, depending on the change of mass
density and tension.