How to switch to a tab in mobile Safari without it reloading?
The bottleneck is the available free memory (RAM) on the iPhone.

If Safari starts running out of memory it will flush the contents of older tabs meaning they have to reload when you switch back to them.

If you have too many tabs open, or you simply have multiple tabs that each have a large web page in them, Safari will reload when you switch back to another tab. There’s no fixed limit; it depends on the combined size of the HTML + all assets each web page you have open and on the total number of tabs you have open.

As the available RAM in new iPhone models increases, this will become less and less of a problem.