On this very website, every page load says:
"This site requires you to update your browser. Your browsing experience maybe affected by not having the most up to date version."

For about half a second and then continues on to loading the requested page.

I'm using Firefox 40. Not exeactly outdated, if I may say so.
SS devs, you might wanna fix this. Not only is it downright incorrect, it also causes an ugly flash of content when navigating from page to page.

A form of the age old 'FOUC' issue I think. I've noticed it too. Depends on the speed of the connection, which on the opposite side of the world here from NZ is quite slow - so much more of an issue than it probably is for SS devs.

I'm in Europe, and judging from the spelling used on the SS website, I'm assuming SS is situated in the US. Usually EU<->US connections are pretty decent, so it's fairly safe to assume many other folks would be experiencing this as well. On top of that, there are still people on slow connections regardless of distance.

I'm on 100Mb fiberoptic, so any slow website immediately stands out as such ;)

But, getting back to the problem... A competent web developer can work around this FOUC issue. For example by reversing the check: "any browser is recent, until proven outdated". You could also say competent web development doesn't need such message in the first place.

Yeah don't get me wrong, it's mildly annoying and there are a bunch of other things I'd do to improve the UX of the site.
But complaining about it is generally not my opening gambit to a volunteer led support community.

I'm also in Europe, and the site loads slower than a wet week for me.

As a side note: Being from New Zealand it's supposed to be English English, but there is at least one American employee, guest bloggers, and general proliferation of 'Americanese', which (coupled with a multitude of nations that learn American English by default) means the spelling consistency can sometimes waver. But dialects of English have nowt to do with the physical location of a server - which I imagine is probably on AWS. Which centre it's based at though, I cannot say. Nor can I explain the slowness of the site as a whole. But it is irksome, yes.

pretty sure it has nothing to do with silverstripe and is just some junk in this particular site's theme / javascript.
maybe the css doesn't load fast enough to hide it.

I don't see it anywhere where it was implied that the software was at fault. I believe the appeal was made to SS Ltd. - who do run & maintain the site (server, software, and theme 'n all).

I believe that's exactly what I said :)