Depending on the site, 50-100 lines per order is about the soft limit for orders because of performance. For 200 lines, we would expect it to be very slow.
There's no real solution for this. It's something we're aware of but there isn't much we can do without significantly rearchitecting the way 'live' sales orders are handled.
What I have seen some customers do is either:
1. Use the quote to sales order functionality, as I believe quotes can handle more lines than live sales orders
2. Create a custom service that allows the customer to upload a CSV that converts it to a sales order in the backend, completely sidestepping the checkout process.
Both of these have their own additional issues as you're basically sidestepping the entire checkout functionality.
And no, buying additional SCP licenses or a higher service tier will not help.