Why is ContainsOnly so slow for checking a list is full of zeros?

I’m on Windows 10 Mathematica 12.1.1.

I would expect allocating a big ConstantArray and comparing would be slower but apparently it is not:

RepeatedTiming[RandomReal[1, 10000] == ConstantArray[0, 10000]]
(* {0.0000589, False} *)

RepeatedTiming[ContainsOnly[RandomReal[1, 10000], {0}]]
(* {0.00309, False} *)

This becomes even worse if I add SameTest->Equal which the documentation states allows some numerical tolerance:

RepeatedTiming[ContainsOnly[RandomReal[1, 10000], {0}, SameTest -> Equal]]
(* {7.92, False} *)

Why is the above more than ~100,000 times slower than the code that allocates a ConstantArray with a large ByteCount?! What is going on behind the scenes in ContainsOnly that’s more expensive than allocation?

Admittedly, in this much bigger case involving sparse arrays ContainsOnly is faster, so I’d guess the allocation overhead eventually catches up:

RepeatedTiming[SparseArray[999999 -> 1] == ConstantArray[0, 1000000]]
(* {0.00212, False} *)

RepeatedTiming[ContainsOnly[SparseArray[999999 -> 1, 1000000], {0}]]
(* {0.0000169, False} *)