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 large file sizes.
Subsequent form viewers get a performance boost
When a form is viewed once, it gets loaded into a cache so that subsequent viewers of that form from the same country have significantly better performance compared to the first person who viewed the form.
This cache is cleared every time the form editor makes a change to their form (so that the public always see the latest version of the form).