But, is only me, or are the sentences ambiguous?
Can it be construed as a) She was not considered a fast enough runner to join the team, and b) She did not act fast enough, and so there were no openings left for her in the team.
Both are correct. What exactly are you worried about? morzh is correct, they could mean she could run fast enough or she signed up fast enough (or not) That is an English limitation. But, the answer to your question is that they are both right.