The article goes into CPU usage (which seems awesome...) but I don't see anything about the buffer sizes for either. It just mentions them as knobs that can be twiddled.
The table mapping compression levels to window sizes is at https://github.com/facebook/zstd/blob/15a7a99653c78a57d1ccbf.... The first element is the window size. I'm pretty sure you raise 2 to that power to get the window size. As you can see, level 1 is 2^18, which is 256KB - far larger than the minimum of 1KB. Lowering it will make compression faster at the expense of compression ratio.
For comparison, zlib's max window size is 32KB. That's one reason its compression ratio is so limited.
http://fastcompression.blogspot.com/ see under "Custom window sizes" section