Gzip compression not working properly

Something appears to be interfering with Gzip Compression. We have checked to ensure that the required modules are indeed enabled on the server, and have double-checked php.ini & .user.ini files to ensure the correct rules are present.

But while https://checkgzipcompression.com/ confirms Gzip is indeed enabled on the site, GTMetrix does not see it as enabled. For example, I have added a robots.txt file with the sitemap.xml URL in it to action the Robots.txt recommendation in SmartCrawl, but a fresh SEO Checkup scan still does not reflect that.

All caches have been cleared before each attempt at a fresh scan, and plugin conflict test performed twice.

We are especially concerned with category archives that are extremely slow to load. Most taxonomies seem to load normally (subcategories, product listings, pages) but our 4 parent cat pages: CNC Mill, CNC Lathe, Fabrication and Other Machines take upwards of 90 seconds to load