What is the speed impact of gzip on files for HTTP transfer?

Question

I know that gzipping files before sending them across the network saves bandwidth, and for static files that can be cached, it is not a significant impact on server-side CPU usage.

But what about the client? They have to gunzip whatever files are sent, which will take CPU time. Additionally, I'm worried that the entire file must be received and gunzipped before any parsing can take place.

This strikes me as odd because I see two scenarios:

1) client has fast internet   -->   gzip is relevant 2) client has slow internet   -->   gzip prevents partial parsing 

Clearly the exact speed-up (or slow-down?) will depend on exact circumstances of the files being transferred and the client. However, I'm curious what the time cost (or how can I measure the cost) on the client-side?

Answer 1

They have to gunzip whatever files are sent, which will take CPU time.

Perhaps, but the CPU time spent on decompression is extremely small compared to all the other things going on when loading a page (parsing, styling, rendering, scripting).

I'm worried that the entire file must be received and gunzipped before any parsing can take place.

Don't worry, gzip is a "stream" of data and the complete file is not required to begin decompression/parsing.

Specifically I want to know how I can gauge how much time is lost because of gzipping.

Here is an interesting article where the author performs the type of test you're describing. The tools are available for download so that you can perform the same tests in your own environment.

The author concludes:

I guess there are very few cases where you shouldn’t use gzip your content. If your typical page is less than 100 bytes then gzipping it could hurt the client’s and the server’s performance. But no website —except maybe a few web-services— serves pages with a typical size of 100 bytes or less. So there’s no excuse for serving uncompressed HTML.