Allison,
This depends on what type of dynamic content you want to display. If it is something small, like the user's verbatim entry into one of your form fields, then consider using tokens on your confirmation page to achieve this. You may find that the performance is better than using segmentations. If tokens are not sufficient because you require entire paragraphs, images, etc. to be dynamic then you can look into owning the form landing page yourself. This would be more complicated to implement but if you are interested -> in this case, when the user fills out your form (you would own the landing page and the form) you would leverage Marketo's
Muchkin API to associate the information entered in the form with an existing lead in Marketo (or create a new lead if one doesn't already exist). This way, not only will Marketo have the information entered in the form but from there you could also direct the user to your own confirmation page where you would show appropriate content using some basic JavaScript that would pivot off of the user's form entry (you would need to store the entered form values somewhere to your site locally and then pivot off of that). These two options are probably your best bets if segmentations truly won't work due to time constraints.
Hope that helps!
Justin