Slow basement drain

[FlowBmore]

Bathtub drain in the basement that is extremely slow until it gets a certain amount of water in it. At that point it speeds up and behaves normally.
Tub is freestanding and connects to floor with a flexible hose.
There is a toilet and sink in the same bathroom that behave normally.

Is this a venting problem?

 

[MicEd69]

Need a picture of your "flexible hose". Is this flexible hose acting as your P-trap? Do you know what the piping looks like under the basement floor?