Q: Can a short peer review process indicate manuscript rejection?
I had submitted a manuscript to a journal. First day, it showed "With editor," then after one day, it showed "Under review." After that, within one day it showed "Ready for Decision." Is this a negative signal?
If the status changed to "Ready for Decision" within a day, it is indeed an unusually quick review. Even rapid publication journals would take at least a week to complete the review process. There is a possibility that this journal uses the status "Under review" to refer to the initial screening that the editor conducts before sending the paper out for peer review.
The fact that the status did not change to "Reviewer assigned" or "Reviewer invited" also seems to indicate that "Under review" probably refers to the internal editorial review or screening. If that is the case, a change to "Decision in process" might indicate a rejection.
Another concern that I have, and which is slightly more worrying, is whether the journal is a reputable one. Some dubious journals have a very quick peer review and decision making process. If that is the case, it could be a problem for you. I hope you have checked the journal's website and verified that it is a reputable journal before submitting your manuscript. If you have not, please do so immediately.
If your paper is accepted meanwhile, write to the journal asking them to withhold publication. Be completely sure of the reputation of the journal before you give them the permission to publish.