API performance is entirely dependent on:
The time required to read the updated table.
The time required to enforce the Security Filter for the updated table.
The time required to compute expressions for the Virtual Columns of the updated table.
If the Virtual Column expressions of the updated table refer to other tables, you must include the time to read those tables, enforce their Security Filters, and compute the expressions for their Virtual Columns.
The time required to perform Workflow rules that are triggered for the updated table.
The time required to re-write the updated table.
As a result, API performance is almost entirely application dependent.
The best way to understand API performance is to use the Performance Profiler. You can open the Performance Profiler by going to Manage > Monitor > Performance Profile in the Editor. Then look at the REST API Invoke results in the Performance Profile.