How to manage high traffic sites with Carthrob. Can it be done or has it meet it's match?

We use EE with CartThrob and Transcribe for a high traffic website. We have a huge, 96 CPU dedicated database server, but it’s having trouble keeping up with orders, as we have close to 19 million rows in the database now, with 800,000 plus in the channel_data and channel_titles tables.”

Looking for ideas on how to better optimize, archive etc to get this manageable.