> the B2 API is much slower than S3.

This is "generally true" for 1 upload thread. We aren't even sure what Amazon is doing differently, but they can be a little faster in general for 1 thread (some people only see 20% faster, some see as high as 50% faster, might be latency to the datacenter and where you are located).

As long as you use multiple threads, I make the radical claim that B2 can be faster than Amazon S3. The B2 API is slightly better in that we don't go through any load balancers like S3 does, so there is no choke point. What this means is that in B2 40 threads are actually uploading to 40 separate servers in 40 separate "vaults" and none of the threads could possibly know the other threads are uploading and it does not "choke" through a load balancer. This was all designed originally so that 1 million individual laptops could upload backups all at the same time with no issues and no load balancers. And it works great every day.

Practically speaking, for most people in most applications, this means both Amazon S3 and Backblaze B2 are essentially free of any limitations. If you aren't using enough of your bandwidth, spawn a few more threads (on either platform) and soak your upload capacity. But in full disclosure, if your application is only single threaded, yes, B2 tends to be 20% slower for that 1 thread.