"It would have been half an hour, suppose she was doing faster."? Not good.
A conjunction joins two sentences to make one sentence. 'con' = with + join. Suppose is not a conjunction. It is the imperative of the verb 'suppose' here = 'make the assumption that'
Suppose [here the condition] then [here the result] (You can put [result] before [condition])
Suppose she did it faster, (then) it would only take half an hour.
Your condition has 'was doing faster'. I'll rewrite that (you can change 'supposing' for 'suppose':
[Supposing she were doing it faster], (then) [it would only take half an hour.] Or using the Perfective:
[Supposing she had been doing it faster,] (then) [it would only have taken half an hour.] Swap the condition and the result around:
[It would only have taken half an hour,] [supposing she had been doing it faster.] Remove supposing:
1) It would only have taken half an hour. 2) She had been doing it faster. Both seem a bit incomplete, and 2) has become something different: not a hypothesis any more, but a simple statement of something in the past, whereas your original intention was to hypothesize on a result dependent on her being speedier, which she clearly wasn't. This is the realm of the subjunctive. 2) is not the same as '(supposing) she had been doing it faster', for the simple reason that she hadn't been doing it faster, or you wouldn't be discussing possibilities connected with more speed.
