can netsuite be made to send data directly (transaction by transaction) to a a 3rd party integration instead or sending the data in batches?

Yes, via user event scripts, for example. These are server-side scripts that can be configured to fire when actions on records (create, load, edit, …) take place in a given context (via API, via UI, CSV upload, script, …).

But wouldn't that slow down the system as this would be happening for each transaction every time a transaction is submitted?

It would. You could just use a scheduled script instead