It was supposed to be a quick arc, but the escape situation got messy, and there's a reason for it.
Luffy & co. fulfilled their objectives (poneglyph copies, rescuing Sanji, saving Germa, etc.) 'without a scratch'.
That left BM 'image' damaged, and she'd have felt as a regular 'one arc' threat to the SH if they had succesfully escaped after the Whole cake Chateu fell apart.
Therefore, Oda had to show the 'enormous threat' that BM represents in the 'run away' second-half of the arc, so that the reader recognises her as a bigger threat than the regular-arc villain.
Still, it'll be no more than 6-7 chapters left to finish the arc, which leaves us the Reverie for late November - early December and Wano for late January tops. It's not that big a deal, tbhonest.