differences in TTFB in safari vs chromium based browsers

[adardesign]

While monitoring multiple websites using web-vitals I noticed that there is a huge consistent difference in TTFB between browsers I checked separately mobile vs desktop traffic Shouldn't TTFB measure the backend and network which should be the same across browsers.

[tunetheweb]

TTFB measures more than just the backend time. It measure the time from when the click happens, through any redirects, and SSL/TCP set up, the resource request, until the first bytes are returned.

Therefore it is hugely influenced by the users networks and devices, not to mention HOW they get to your site (directly, from an ad with redirect, from a link using a link shortener with a redirect?), and not just your backend performance time.

Doing a quick query from the rumarchive public dataset on BigQuery:

SELECT DEVICETYPE, USERAGENTFAMILY, AVG(TTFBAVG) as AVG_TTFB, SUM(BEACONS) as BEACONCOUNT
FROM akamai-mpulse-rumarchive.rumarchive.rumarchive_page_loads
WHERE (DATE BETWEEN '2023-12-01' AND '2023-12-31')
GROUP BY DEVICETYPE, USERAGENTFAMILY
ORDER BY BEACONCOUNT DESC

You see that Safari often does have a lower TTFB than Chrome:

DEVICETYPE	USERAGENTFAMILY	AVG_TTFB	BEACONCOUNT
Mobile	Mobile Safari	886	1,488,156,723
Desktop	Chrome	929	1,305,147,671
Mobile	Chrome Mobile	961	784,312,202
Mobile	Mobile Safari UI/WKWebView	726	544,977,427
Mobile	Chrome Mobile WebView	867	416,299,416
Desktop	Edge	902	333,017,611
Desktop	Safari	802	206,702,655

I imagine this is more reflective of the types of users and networks that use Safari than the browsers themselves though. Limiting it to just GB/UK for example reduces these differences, but even then doesn't account for socioeconomics differences likely affecting network speed.

Anyway, point is this is not unexpected and you can probably analyse your data more to find similar reasons.