well since it might eventually need to handle 200k requests, you have plenty time to identify any issues before they might be occuring.
I would suggest writing clean and efficient code that does no things it shouldn't be doing.
Additionally you might want to think about the form validation. Do you want to check the incoming data for proper formatting? if that's less important, you might want to swap that checking to javascript, which is less secure but calculates in the user browser (free resources for you! 😉
do note that users can quite easily circumvent javascript. The question is do they want to, and who will they be hurting if they do. If they hurt no-one or only themselves, then why not go for javascript validation.
What happens with the form? Simply storage in a DB? I'd say keep the query clean, the table structure logical (probably a single table) and don't do stuff you don't need to do.
Since it's an old site, I doubt optimizing this stuff will save you from hitting the first bottleneck (which I suspect will be found in old and possibly less optimized code).