Compared to the second-generation Epyc processors that Google is using, the first generation has lower clock speeds, can execute fewer instructions per clock (particularly in terms of floating-point operations), has substantially less cache, and has a more complicated memory topology that can negatively impact the performance of workloads that aren't NUMA-aware.

In short, your experience with AMD in AWS isn't relevant to Google's offerings.

I don't work with Rails anymore but the last Rails app(single threaded, Unicorn) I worked with raw CPU compute was not the bottleneck usually as with most CRUD apps time was mostly spent in I/O. This effect was so pronounced that I had set up most scaling groups on M or R instances as the memory used by the gems was the bottleneck on the number of Rails processes I could spawn in the box without exhausting resources. However I do remember even if CPU was not the bottleneck, moving to a processor with a better single thread performance did improve the median response time at the cost of making the same request throughput costlier.