The Speed Cost of Serving Critical Assets from Multiple Origins

The overhead of serving critical assets from multiple origins and how to fix it.
Dec 28, 2019

For every resource that the browser fetches on the web, it has to first set up a connection with the server. This typically involves DNS lookup, TCP initial connection and SSL handshake (for https connections). The browser receives the file only after the connection is setup. So, the total time in which the browser finishes the request also includes connection setup time.

Let’s understand how this time can cause notable delay in page loading experience and how to address it:

Time to setup a single new HTTPS connection

Setting up a single new connection involves more than one round-trips of protocol specific data packets between the browser & the server. This time is largely latency dependent. It can be below 100 msec in fast low-latency envrionments or 2 sec for mobile devices on slow networks. For (TLS 1.2, served from India), I have observed it to be ~700 msec for mobiles with Fast-3G connections from India.

New connections & multiple origins

Loading a web page requires downloading multiple files (html, css, javascript, images, fonts, etc). If all these files were served via a single origin end-point, the initial connection established would be reused for fetching rest of the files. And the connection setup delay would be a one-time affair in the page loading journey.

However, it is common for websites to serve HTML from base origin (like, static assets from CDN (like, font files from fonts provider (like Everytime a file from a new origin is requested, the browser has to first setup a connection with that origin.

So, if your webpage requires files from three distinct origins to display anything, browsers on your visitors’ mobile device may require 1-2 seconds just for the initial connection setup. And, we’re not even considering size of your files or your server response speed yet. More the number of origins to connect to display your page, longer the delay.

Measuring connection setup overhead for a URL

Any time that browser is lazy & waiting for connection setup to happen is costly and we should try remove / minimize it. But, before minimizing, we need to measure it and that requires us to understand when & how browser requests new connections.

The browser requests a new connection to an origin as soon as it knows it has to fetch a resource from that origin. But, this can often be sub-optimal. Let’s understand this via Google Fonts. Using Google Fonts requires you to embed a snippet that loads CSS file from Once the browser downloads this CSS, it discovers that the font-files need to be downloaded from the origin So, the time to display text in the designated font will include the time to setup three connections (to base HTML origin, and

Time spent by browser waiting for connection setup
Time spent by browser waiting for connection setup

The overhead of new connection setup may not be as bad as exhibited above in all cases since the browser may often setup connections in parallel for resources to be downloaded in parallel:

Time spent by browser waiting for connection setup - II
Time spent by browser waiting for connection setup - II

The most accurate way to know how much your URLs are impacted by new connection setup cost is to actually measure it via WebPageTest. It is critical to select the location, device & network conditions that resemble majority of your visitors.

Addressing Connection Setup Overhead

Here are a couple of ways to eliminate or reduce the delay from connection setup during web page loading:

  • Preconnects
    Pre-connects are browser hints that allow us to tell browser we’ll need connection to a certain origin later. This enables browser to setup connection as early as possible so that it doesn’t have to spend this time waiting later. How browser handles preconnects
    How browser handles preconnects
    Preconnects should be leveraged only for origins serving critical files. Since these aren’t complicated to apply, there is no reason pre-connects should not be leveraged. You can also test how much preconnects will help improve your website speed using this technique.
  • Reduce Distinct Origins
    Pre-connects can minimize lazy time by setting up connection early but they cannot completely eliminate it. However, reducing the number of distinct origins used to serve critical assets eliminates setting up new connections. This may be non-trivial in some cases and may require setup / hosting / configuration changes. However, it is highly recommended as the long term solution to a more reliable & performant way to serve your web page resources.

    Eliminate additional connection setup cost with self-hosting
    Eliminate additional connection setup cost with self-hosting


Connection setup overhead is often overlooked because it is not substantial on fast networks & non-existent for repeat visits (due to keep-alive). However, it can be significant for new sessions on mobile networks. And it’s impact multiplies with larger number of origins to serve critical assets. Measuring it from a visitors’ perspective helps website owners realize the slowness this causes. Applying preconnect browser hint can help reduce it’s impact. However, to completely eliminate it, we should strive to serve all our critical files from a single origin endpoint.

Punit Sethi has been working with large e-Commerce & B2C websites on improving their Site Speed & Scalability. He frequently tweets here.
Punit Sethi