Presumably the problem is they are not using the (free) Adobe Reader - the only program that provides the functionality required.
As HomeJames notes the PDF "Submit" button not working would be that users are not using the free Adobe Reader to fill out and submit the form.
In the web browser, are you using the PDF or the web/HTML form? If it is a PDF it is the same issue, not using Adobe Reader, the PDF should be downloaded and filled out using Adobe Reader.
If you are using a web form in the browser, what is the performance issue?
If by webform you mean the version that is generated in Formcentral for embedding onto a website, your question is simple to answer.
The problems I have described occurs when using this webform, which I have generated for embedding. I have found that it is easier to email the pdf version to the customer, because then they can open it in Adobe Reader.
I have aslo found that the webform works in Safari and Explorer. In Firefox it opens without functioning fields, but firefox opens a line stating that the document is not shown correctly, and they give option to reopen in program of choice. If doing this in Adobe Reader, it works.
In google chrome it just does not work...
Maybe the webform should be rebuilt, or I should just forget about it and arrange a pdf download button instead?