Optimize Existing Code

This is a simplified version of a GitLab-style backend service that summarizes merge request activity for a project: which merge requests are open, which have gone stale, who's been approving things, and nudging stale authors by email. On a busy self-managed instance with a lot of history, calling any of these methods takes noticeably longer than it should.

class MergeRequestService
  def initialize(project)
    @project = project
  end

  def open_merge_requests
    MergeRequest.all.select { |mr| mr.project_id == @project.id && mr.status == "open" }
  end

  def stale_merge_requests(days = 14)
    open_merge_requests.select do |mr|
      (Time.now - mr.updated_at) / 86400 > days
    end
  end

  def approvers_summary
    summary = {}
    open_merge_requests.each do |mr|
      mr.approvals.each do |approval|
        user = User.find(approval.user_id)
        summary[user.username] ||= 0
        summary[user.username] += 1
      end
    end
    summary
  end

  def notify_stale_authors
    stale_merge_requests.each do |mr|
      author = User.find(mr.author_id)
      NotificationMailer.stale_mr_warning(author.email, mr.title).deliver_now
    end
  end
end

What optimizations can be done with this code? Explain where the cost is actually coming from.