The synchronous aspect is a problem, although not a huge one. The issue is mitigated to a certain extent by excluding non-customer facing research & testing components from the review requirement. Also, the architecture of the system is such that people tend to work very independently, with little-to-no overlap, so reviews can be postponed with little impact on velocity. As our team grows in size, we could also think about instituting a rotating code reviewer responsibility, with team members taking it in turns to act as the designated reviewer for the day.