When you are on a website, eBay for example, click the Safari top menu. You will see “Settings for this website”. Select that and you will see the option to block pop-ups. There is no longer a global ‘block popups’ option. Why? Because some sites like online retailers require popups to function properly. Each website can be set accordingly.
That being said popups have absolutely nothing to do with websites crashing or reloading. You might try clearing your browser caches. Safari->Preferences->Advanced and check ‘Show Develop menu in menu bar’. The Develop menu will appear at the top memu bar. From there you can select ‘Empty Caches’ or just use the keyboard shortcut option/command/e to clear your browser caches. See if that helps.