It’s probably negligible, though I expect it depends on how many fields your form has, and how efficient the form-generating code is. And how busy the site is. If it starts to appear to be running slowly, you have the option to change one of your forms to another method and see if it actually makes any difference.
It’s the kind of thing I think about, having come through computing when we were excited to get our first 20Mb hard disk and 286-based PC, but these days I would imagine few others do.
I guess the least impactful might be no “dynamic” at all. Every time a file needs changing it’s “manual” hard coding — IMHO, can be OK for things that don’t change more often than annoys you
The opposite exteme might be a script that bogs out to the point of hanging the browser. Once code is in place it simply needs to be triggered to run — IMHO, good for when things change more often than you care to deal with all the time. Of course you want to not bog or hang.
For example, a CMS may have a database field for the site name. The template(s) may run a query every page and then display the name. Wasteful? Yes, not much perhaps. Indeed, almost assuredly insignificant, but wasteful just the same assuming once a site name is chosen it isn’t likely to change any time soon.
Maybe putting together a script that caches a result instead of running a query unless there’s been an update?
Note of caution, even though caching can be a very good solution in case your forms really are slow, it’s not a good solution if they are not slow. Since caching brings some problems of its own. Since caching brings some problems of its own.