The original manuscript is not extant here. In the printer’s manuscript, Oliver Cowdery initially wrote “go against those dissenters”; then he supralinearly inserted speedily without any change in the level of ink flow. He was undoubtedly restoring the reading in 𝓞 since either reading works and there would have been no need to emend the text here. Parhoron used the word speedily earlier in his epistle: “therefore come unto me speedily with a few of your men” (Alma 61:15). Apparently he wished to emphasize the need to act speedily, just as Moroni in his epistle had emphasized the need for speedy action, although more as a threat to Parhoron.
Thus speedily is quite appropriate in Parhoron’s reply to Moroni.
Summary: Accept in Alma 61:17 the corrected reading in 𝓟, “and we will go speedily against those dissenters”; Parhoron seems to emphasize the word speedily in his reply to Moroni, although Moroni had used the word more to threaten Parhoron.