Actually excited by this. I was benchmarking S3 vs B2 vs others 2 years ago and I had to give up on B2 because its implementation for performance was so much more difficult. (88 lines vs 36 lines for all others in Ruby)
It was just super hard to make the code perform well. Like you have to manage client sessions on your side and chose optimizations on your side. Like you have to spread things manually. Which is hard to do. Whereas S3 is maximising your bandwidth with no custom code required. It's not really S3 compatibility that was needed but B2 API wasn't good.
There is a price/performance trade-off: B2 has higher request latency than S3, no matter where you are (my experience), but they also are 5x cheaper on storage costs, 10x cheaper on download bandwidth, and have no price gimmicks like minimum object sizes or minimum object lifetimes like many other services (S3 IA for example).
To make up for B2's request latency it is more important to issue requests from multiple threads, especially for short-running requests like removing files.
Another key difference is that B2 always uses SSL whereas S3 can be accessed without SSL with little security impact because each S3 request is individually signed with a secret key. Setting up an SSL connection is more overhead, so another key to performance is to reuse connections.
Both of these suggestions apply to S3 as well, just more to B2 because of the latency difference.