hint that that checks if a page passes a set performance budget
npm install @hint/hint-performance-budgetperformance-budget)> A web performance budget is a group of limits to certain values
> that affect site performance that should not be exceeded in the
> design and development of any web project. This could be the total
> size of a page, size of images you are uploading, or even the number
> of HTTP requests that your webpage generates.
[keycdn - web performance budget][keycdn-wpb]
As of January 2018, the average size of a website is 3,545kB:
![image][average site size]
Although the global average connection is 7.2Mb/s (check [_Akamai's
state of the Internet 2017_][state of the internet]), _"no bit is
faster than one that is not sent"_ (quote by [Ilya Grigorik][faster
bit]). Web developers need to be mindful not only about the size of
their sites, but also the number of requests, different domains,
third party scripts, etc. The time required by a browser to download
a 200kB file is not the same as 20 files of 10kB.
This hint calculates how long it will take to download all the
resources loaded initially by the website under a 3G Fast network
(but that can be changed, see ["Can the hint be configured?"][can be
configured] section). If the load time is greater than 5 seconds,
the hint will fail.
To calculate the final load time, some assumptions and simplifications
are done. While the real numbers might be different, the results should
provide enough guidance to know if something needs more attention.
The reason for using predefined conditions and assumptions are:
* Guarantee consistent results across runs. If a website serves the
same assets, the results should be the same.
* Show the impact in load time of each transmitted byte with the goal
of reducing the number and size of resources downloaded (first and
third party).
The simplified formula to calculate the time is:
``text`
Time = (total number of requests * RTT) +
(number of different domains * RTT) +
(number of different secured domains * RTT) +
(number of redirects * RTT) +
(total number of requests * TCP slow-start phase) +
(total size of resources / bandwidth)
This is the list of things considered:
* Everything is a first load, no values are cached, and no connections
are opened.
* [RTT (Round-Trip Time)][rtt] is fixed and changes depending onDNS lookup
the configured network. It assumes all servers respond instantly
and in the same amount of time.
* [][dns lookup]: Every hostname resolution requires 1TCP handshake
RTT, imposing latency on the request and blocking the request while
the lookup is in progress.
* [][three-way handshake]: Each request requires a newTCP slow-start phase
TCP connection. TCP connections require 1 RTT before starting to send
information to the server. There's no connection reuse and the maximum
number of connections to a domain (usually 6) is ignored.
* [][slow-start phase]: The values used tocwnd
calculate the duration are:
* : 10 network segmentsrwnd
* : 65,535 bytes (no TCP window scaling)segment size
* : 1460 bytesTLS handshake
After this phase, the full bandwidth of the connection is used to
download the remaining.
* [][tls handshake]: New TLS connections usually requireTLS False Start
two roundtrips for a "full handshake". However, there are ways of
requiring only 1 RTT like [][tls false start] andTLS Session Resumption
[][tls session resumption]. This hint assumesHTTP/2
the optimistic scenario.
* Redirects: We simplify the redirects and assume the connection is
reused and to the same server. If the redirect is to another domain,
the penalty will be even greater.
* The server doesn't use [][http2].
You can change the type of connection and/or the target load time
in the [.hintrc][hintrc] file, using something such as
the following:
`json`
{
"connector": {...},
"formatters": [...],
"hints": {
"performance-budget": ["error", {
"connectionType": "Dial",
"loadTime": 10
}],
...
},
...
}
The possible values and the associated speeds for connectionType are:
| Value | In | Out | RTT |
| -------|----------|----------|-------|
| FIOS | 20 Mbps | 5 Mbps | 4ms |
| LTE | 12 Mbps | 12 Mbps | 70ms |
| 4G | 9 Mbps | 9 Mbps | 170ms |
| Cable | 5 Mbps | 1 Mbps | 28ms |
| 3G | 1.6 Mbps | 768 Kbps | 300ms |
| 3GFast | 1.6 Mbps | 768 Kbps | 150ms |
| DSL | 1.5 Mbps | 384 Kbps | 50ms |
| 3GSlow | 400 Kbps | 400 Kbps | 400ms |
| 3G_EM | 400 Kbps | 400 Kbps | 400ms |
| 2G | 280 Kbps | 256 Kbps | 800ms |
| Edge | 240 Kbps | 200 Kbps | 840ms |
| Dial | 49 Kbps | 30 Kbps | 120ms |
loadTime needs to be a number greater than 1 and indicates the time
in seconds to load all the resources.
The default values are:
* connectionType: 3GFast
* : 5
This means that if the user changes the connectionType but not theloadTime, the hint will use 5 as the target.
* Any combination of sizes, redirects, requests to different domains,
etc. that make the site load after 5s on a 3GFast network using the
established formula.
* Any combination of sizes, redirects, requests to different domains,
etc. that make the site load in or under 5s on a 3GFast network
using the established formula.
To use it you will have to install it via npm:
`bash`
npm install @hint/hint-performance-budget
Note: You can make npm install it as a devDependency using the--save-dev parameter, or to install it globally, you can use the-g parameter. For other options see npm's
documentation.
And then activate it via the [.hintrc][hintrc] configuration file:
`json``
{
"connector": {...},
"formatters": [...],
"hints": {
"performance-budget": "error",
...
},
"parsers": [...],
...
}
* [High Performance Browser Networking][hbpn]
* [Can You Afford It?: Real-world Web Performance Budgets][can you afford it]
* [Setting and Calculating a Web Performance Budget][keycdn-wpb]
* [The Cost Of JavaScript][cost of javascript]
[average site size]: https://chart.googleapis.com/chart?chs=400x225&cht=p&chco=007099&chd=t:1818,70,98,504,120,851,27&chds=0,1818&chdlp=b&chdl=total%203545%20kB&chl=Images+-+1818+kB%7CHTML+-+70+kB%7CStylesheets+-+98+kB%7CScripts+-+504+kB%7CFonts+-+120+kB%7CVideo+-+851+kB%7COther+-+27+kB&chma=|5&chtt=Average+Bytes+per+Page+by+Content+Type
[can be configured]: #can-the-hint-be-configured
[can you afford it]: https://infrequently.org/2017/10/can-you-afford-it-real-world-web-performance-budgets/
[cost of javascript]: https://medium.com/dev-channel/the-cost-of-javascript-84009f51e99e
[dns lookup]: https://www.cloudflare.com/learning/dns/what-is-dns/
[faster bit]: https://hpbn.co/building-blocks-of-tcp/#tuning-application-behavior
[hbpn]: https://hpbn.co/
[http2]: https://hpbn.co/http2/
[keycdn-wpb]: https://www.keycdn.com/blog/web-performance-budget/
[rtt]: https://hpbn.co/primer-on-latency-and-bandwidth/#speed-of-light-and-propagation-latency
[slow-start phase]: https://hpbn.co/building-blocks-of-tcp/#slow-start
[hintrc]: https://webhint.io/docs/user-guide/configuring-webhint/summary/
[state of the internet]: https://www.akamai.com/us/en/multimedia/documents/state-of-the-internet/q1-2017-state-of-the-internet-connectivity-report.pdf
[tcp handshake]: https://hpbn.co/building-blocks-of-tcp/#three-way-handshake
[three-way handshake]: https://hpbn.co/building-blocks-of-tcp/#three-way-handshake
[tls false start]: https://hpbn.co/transport-layer-security-tls/#enable-tls-false-start
[tls handshake]: https://hpbn.co/transport-layer-security-tls/#tls-handshake
[tls session resumption]: https://hpbn.co/transport-layer-security-tls/#tls-session-resumption