That depends on what's going on wit the page itself. May we have a look? There's always a workable solution that can be put in to place.
That all depends on whether the browser has recently accessed the same jquery file. You can for example reference a google-hosted version of it, which many other people use too, increasing the likelihood that visitors will have already cached it.
Because you cannot guarantee that the caching will occur, you should put some thought in to that delay.
Loading the library from the head means that the delay will occur before your page loads, resulting in a longer delay for your user before they get to see your page load.
Loading the library from the end of the body means that the page appears to load much faster, because the delay occurs after the page has rendered itself.