I want to pump some data into several different objects very quickly. I can't use the bulk API because I need the Id's back to update other records in other objects. What is the maximum number of simultaneous connections I can use?
You should use the SOAP API to push the data into Salesforce. You can create a maximum of 10 sessions using a single login. I would suggest that you create multiple user ids and use multiple threads each with their own login user id to connect to Salesforce and then push data into the object. Salesforce support also mentioned that performance will be determined based on if the object references other records in the same object and in different objects. Also, if the object has sharing rules, that could also affect performance.