Brotli compression performance improvement

[SachaMorard]

Is your feature request related to a problem? Please describe.
Brotli compression is slower than other encoding algorithms

Edgee adds a header to the response which indicates the compute duration (x-edgee-compute-duration)
When an upstream server responds a gzip compressed response, the compute duration on my local env is okay (~10ms)
When an upstream server responds a brotli compressed response, the compute duration on my local env is too slow (~120ms)

The slowdown seems to originate from this part:

Some("br") => { // handle brotli encoding
    // q: quality (range: 0-11), lgwin: window size (range: 10-24)
    let mut encoder  = CompressorWriter::new(Vec::new(), 4096, 11, 24);
    encoder.write_all(new_body.as_bytes())?;
    encoder.flush()?;
    encoder.into_inner()
}

Describe the solution you'd like

• Test if brotli compression settings can help
• Look at the strategies and solutions used by other proxies
• Test other libraries

[asolimando]

Here are some figures for x-edgee-compute-duration on my machine with different setups:

• no compression: ~3ms
• brotli (quality 5): ~9 ms
• brotli (quality 11): ~101 ms <-- that's the current hard-coded value
• gzip: ~9 ms

So already by lowering the quality to 5 we have a performance closer to that of gzip, of course a couple of runs without proper benchmarking isn't conclusive, but it already suggests that there is room for improvement by changing some of the default parameters (with #97 in place it would probably be easier to have better figures).

Regarding what other reverse proxies do:
Nginx allows to tune several brotli parameters, including quality/compression level, buffer size, window size and the minimum payload size.

Additionally, not all MIME types equally benefit from compression, so we have an additional parameter for the MIME types for which one wants brotli encoding to be activated.

In summary, a reasonable starting point is to lower the compression quality parameter, and possibly add all or some of these configuration knobs to the Edgee configuration to make it tunable.