I enable profiler
SetEnv MAGE_PROFILER html in my
conf file for magento 2.
Is there an easy way to profile Magento 2 API REST calls.
I would recommend to check (even as a trial option) solutions like:
Those tools will show you exact traces and timing of each separate PHP Trace that is involved within any operation/transaction (not limited to API calls)
For Database related bottlenecks - MySQL Slow Query Log can be enabled (*depends on MySQL Version used). This can be used without mentioned performance monitoring tools