If you're experiencing slow performance in editing mode then please consider this: DatasourceUsagesCount request slowing down Experience Editor. This contains a hotfix.
If you're experiencing slow loading of experience editor then you need to follow these steps that were already mentioned
turn off Experience Optimization
turn off "WebEdit....
Please use sc_mode=preview URL parameter:
If you need to open a link in the tab then you need to use target="_blank":
<a href="www.mysite.com?sc_mode=preview" target="_blank">my site in preview mode</a>
Or you can use Sitecore ...