After performance tuning one of our Magento 2 sites, we noticed a drastic slow down in the Magento admin site.
After tediously testing our server configuration (NGINX, Perconna, PHP7), we were able to attribute the slowness to the server configuration. The verified cause of the issue is the CSS merge setting (Admin > Stores > Configuration > Developer > CSS Settings > Merge CSS Files).
To isolate this setting as the cause, I create a stock Magento 2.02 droplet at Digital Ocean from their one-click applications. About 5 minutes later, we’re up and running with a completely stock testing environment. On the stock settings (CSS not merged), it takes the admin Dashboard 3.65 seconds to load on this server (2 CPU, 1GB RAM).
Next, we changed turned on CSS merge. Voila! After clearing the cache to remove the alert message, the admin Dashboard page takes 33.46 seconds to load the page.
That’s nearly a 1000% increase, for anyone that cares about the math.
We also saw it take 1.2m to load the admin products page after the default product load.
Needless to say, if you want to enable CSS merge to boost your SEO and Google PageSpeed ranking, you’ll have to either live with the slowness on the admin site or disable CSS merge during your heavy admin activities.
We hope to see this issue resolved in a future release of Magento 2, hopefully soon.
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