Hi,
The TcConnector is already using the rest API. What can be not well optimized is the loading of the Teamcenter response (JSON) to Mendix object, this is what the connector names the "Object Mapping".
Since you already have your test working with the connector, it's easy for you to use my module Mendix Marketplace - Teamcenter Connector Toolbox Samples and replace your call by this one "CallTeamcenterService_JSON_output", which expects the same inputs, but returns the raw JSON and skip the "object mapping". If the performance is still not good, you should look at Teamcenter side. In handling the raw response, you may find this useful "Call_postProcessor_JSON_VariableKeysToArrays".
Alternatives :