... about the performance and efficiency gains one can expect from switching to nginx.
Most performance gains from nGinx are from developments in the last few months or relate only to certain workloads. For average users Apache is easier to use, far more mature, often the more performant for very small workloads, far more broadly compatible and known. Given those factors, people who need nGinx (like us, we are using it right now) tend to be trained admins and know when and why to deploy it and those that just need basic reliability and simplicity will get Apache by default.
Even today where Apache is not the big winner it used to be, it seems like it is still a sensible default.