I believe that the single most important thing a team can do collectively to improve its throughput is to make code review a (or better yet, the) top priority for all team members. This is the most compelling and concise reason I could come up with, so I’ll call it out as blatantly as I can:
Pending code reviews represent blocked threads of execution.
Glen Stanford, formerly engineer and engineering manager at Twitter, has a great writeup about conducting effective code reviews, for both submitters and reviewers.