Just take it file by file. If you are using something GitHub Enterprise or the Bitbucket or GitLab equivalents then just leave comments file by file of your progress in reviewing the PR.
Is there a parallel stream for corresponding changes to test automation? If so that helps tremendously. Are there rigorous automated lint/validation/conformance rules in place, because if so that helps tremendously. With enough automation in place your review should be a quick look line by line for major violations and questions about why some change was needed from the business requirements perspective as answered by code comments or documentation. 1500 line PRs don't have to be painful if the platform is mature.