Shoudn’t it be быстрее ?
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When I search for sentences containing идите, I see these two:
Ну, {{идите}} быстрее.
Ну, идите {{быстрее}}.
in addition to the one you mentioned. So быстрее is definitely an option. However, in Wiktionary, there is a page for быстрей, and it says that it can be the comparative degree of быстро. Similarly, the Wiktionary page for быстро lists both быстрее and быстрей as comparative forms. I also see plenty of sentences with быстрей on Reverso Context.
Maybe a native Russian speaker can tell us more.