Why does she wait so long to open my message and respond?

She asked me out and ever since the date was planned I have sent two messages with practical stuff. Before the date was agreed she responded within 10 minutes to each message. Ever since she asked me out, she doesn't open my message hours later and only responds then. Is she subtly telling me she sees me as a friend or is she trying not to seem too eager? She has shown interest in me, by touching, talking a lot to me, being jealous when I talk with other women, she trying to make me jealous with another man...
Why does she wait so long to open my message and respond?
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