We all know that SharePoint is an ASP based on ASP. NET has a feature is the first user when the first visit will be JIT compiled, which caused the page to open relatively slow; In addition, because the hosting site's application pool will be recycled, re-access after recycling and compile action will occur, so this is why the previous visit is faster, suddenly the second day later and slowly.
Now that you know the problem, the way to solve it is simple; Before the first user accesses, the JIT compilation is done in advance by some means, so that when the user is actually accessing it, the page will be opened more quickly.
We can refer to "SharePoint," or "Application warm-up script", a section of PS scripts set to run automatically every day; After the run we go to visit the site collection, this time the page will open quickly. There's also a gadget on CodePlex. You can also refer to "Spwakeup-wake up your SharePoint and WSS Sites".
Of course this way is from the outside, from within the optimization of the SharePoint site there are many aspects, we will be in the future for everyone to introduce slowly.
Enjoy SharePoint
How to Wake up SharePoint