This is just a post to help other Salesforce devs who have to face the same repeat question time and time again and the same disbelief in the answer, So they can prove to the client that they are telling the truth.
If you are reading this then someone has sent you to this post because you have asked about linking to a third parties data “on the fly” or “in Real time” or “before it opens” on Salesforce and wanting for such data to arrive before showing the page to your users…
the simple answer is this
Salesforce will never make the speed of their website dependent on anyone else!!!
It will never wait for anyone else before opening a page
It will never wait for anyone else before saving a page
It will never wait for anyone else to do ANYTHING
Yes, you can call just about anything with Salesforce, but it will do this asynchronously, so it will make the call to the Third Party, then get on with its own stuff without waiting for an answer.
You just have to write your Salesforce code/page/whatever to deal with the result when it comes back from the Third Party, it’s well documented, there are lots of ways to deal with it: Batch runs, Ajax etc and plenty of neat solutions.. but none of them is “can’t it just wait”