First-time form views vs Subsequent form views
On all paid plans, the first time that a form is viewed, it gets loaded into a country-wide cache so that all subsequent viewers of that form from the same country have significantly faster performance compared to the first time that the form was viewed by someone. This is why the second time you view a form it is faster to display regardless of what device you use.
This country-wide cache of your form makes the form fast to load for anyone else in the same country, and is cleared every time the form editor makes a change to the form (so that the public always see the latest version of the form).
So as long as your form gets at least some traffic per week it will continue to stay in "cached/primed" mode for maximum performance.
Simple forms vs Large forms
Generally the larger and more complex a form is, the slower it will load, and the more compact a form is the faster it will load. If you have uploaded images into the form please also make sure that they are optimised for the web and do not have excessively large file sizes.