Azure Website slow to serve static JS/CSS but not binary

We had this issue, and was able to resolve this with the help of Azure Support Team. The issue was that the slow files would use TransferEncoding: Chuncked. They suggested that we force static compression to get around this issue.

We had to add the following to <system.webServer>:

<serverRuntime enabled="true"  frequentHitThreshold="1"  frequentHitTimePeriod="00:00:20" />

To elaborate on John Tseng's answer: (from here)

As you saw earlier, IIS 7 caches the compressed versions of static files. So, if a request arrives for a static file whose compressed version is already in the cache, it doesn’t need to be compressed again.

But what if there is no compressed version in the cache? Will IIS 7 then compress the file right away and put it in the cache? The answer is yes, but only if the file is being requested frequently. By not compressing files that are only requested infrequently, IIS 7 saves CPU usage and cache space.

By default, a file is considered to be requested frequently if it is requested two or more times per 10 seconds.

So, the reason your users are being served an uncompressed version of the javascript file is because it didn't meet the default threshold for being compressed; in other words, the javascript file was not requested 2 times within 10 seconds.

To control this, there is one attribute we must change on the <serverRuntime> element, which controls compression: frequentHitThreshold. In order for your file to be compressed when it is requested once, change your <serverRuntime> element to look like this:

<serverRuntime enabled="true" frequentHitThreshold="1" />

This will slightly impact your CPU performance if you have many javascript files that are being served and you have users quite often, but likely if you have users often enough to impact CPU from compressing these files, then they are already compressed and cached!

Looks like it was an issue on IIS 8.5 and not only Azure specific.

Now App service upgrade to Windows Server 2016 looks complete and this workaround should not be needed.