Problem:
In a recent site profiling, I found that the output from a D8 Views REST export wasn't getting gzipped.
The Chrome Audit reported that gzipping would reduce the payload size by ~ 2/3 (~69.4 KB).
Solution:
Add the following to .htaccess:
# Force enable compression of any JSON output.
# mod_deflate will only compress if client supports it.
# https://httpd.apache.org/docs/trunk/mod/mod_deflate.html
# Don't use this with reverse proxies, lest the compressed response
# get cached for all clients.
<IfModule mod_deflate.c>
AddOutputFilterByType DEFLATE application/json application/hal+json
</IfModule>