Just want to chime in with my findings.
I'd tried all the fixes. Disabling power saving feature, removing all extensions, resettings safari and clearing all website data.
I booted into guest mode and safe mode and was still getting terrible performance. I was also getting poor performance across all browsers (google chrome and firefox).
I had sophos antivirus installed. As soon as I removed this pages were instantly loading and 10x more responsive. Might be worth a try if you have this installed and are having safari issues. I'm off to find a new anti virus program!
For me the issue was Trusteer Rapport - uninstalled this and everything started working fine.
See also Re: Safari quit unexpectedly while using the librooksbas.dylib plug in - this error message showed up, and Safari stopped working entirely, after I deleted the cache, as suggested at the start of this thread.
Rapport also appeared to stop Firefox (v. 30) working.
It would seem that there are a number of issues that can be causing this problem (websites loading very slowly or not at all, progress bar hanging up and freezing). I tried booting into Recovery mode and repairing permissions and reinstalling Mavericks, several times each, with no luck. Finally, after reading further through this thread, I delved into the Plug-Ins and Extensions in Safari Preferences, and, short story shorter, it turned out to be an incompatible extension called Greenhouse. I uninstalled it, and Safari is zipping along like it should. We'll see how long this lasts—I thought it was fixed after repairing permissions a week ago, only to have the problem return today. But, so far, it looks like things are back to normal. Just my two cents, but that's all it was for me. I would suggest trying the Plug-Ins and Extensions route before going through all the other gyrations, it might just be that simple. Thanks to all who posted here, saved me a call to tech support.
It isnt a setting
Its Safari's stupid power saving tendencies.
Safari is almost completely deprecated. 5 of the ten websites i have to administrate daily wont work in Safari and i get a dialogue telling me that Safari is not supported
It switches off java after a short time in tabs (often i come back after an hour and all buttons such as upload, save etc wont work)
Cant upload an image to a folder in my facebook pages because Safari wont work, it just fails.
I was studying safari development a couple of years ago and joined the developer forums and was reading Apple's guidelines for programming power saving options into Safari to make it seem 5 times more powerful than it really is. Unfortunately for webmasters like me, switching off all plugins and flash and ads and video embeds until they are needed, is a pain in the butt, because all my other browsers (waterfox, chrome) work fast and seem powerful without having to switch off all the functionality
Safari is truly deprecated on many sites.
The reason it is slow to load, is timeout, that if you dont surf constantly, and leave tabs open, Safari auto switches stuff off internally to 'save power'
Its just an old browser that is not being developed for compatibility.
As i said, many websites wont work with it. mailchimp is my newsletter service and i cant complete a newsletter in Safari because the AJAX JSON and Java just wont work in Safari.
I have had Safari as my favorite browser for last 6 years, but now im onto waterfox and sometimes even chrome (which i also hate because its google spyware and forces log in to account to enjoy it to the full.. privacy issue!)
Bt now Safari is truly closing in on Mozilla and Netscape as a browser that does not support the code in many new websites, and doesn't plan to any time soon.
I am amazed that Apple really doesn't take notice of the ever increasing number of major websites that do not siplay or function anymore in safari, and i am constantly arriving on sites where i get the message telling me that i need to use an up to date browser, even though im on latest version of Safari
Its the same loading problem in El Capitan too.
As i said at the start, it has noting to do with settings. Its the way safari (doesn't) works (properly)