Web to Lead and Web to Case via API too slow

Our Web team is still using an older (created in v6.2) custom entry point to create Leads and Cases in Sugar from Web Forms.

They did some benchmarking for using v10 APIs and are not happy with the time it takes to execute the three curl requests: get oauth token, the actual request to create the lead/case, releasing the token.

Also, as far as I understand we can only have one oauth token per username/login at any one time, so wouldn't that be an issue with multiple customers hitting submit at the same time?

I know I must be missing something...

So how do you address performance and multiple people hitting the form at once?

thanks,
FrancescaS