It may normally be difficult to measure actual time consumption of a piece of code in a real target system. This normally requires full trace functionality. However, for Cortex-M processors, good measurements can be achieved by SWD/SWO/SWV.
The link How to measure time with SWO.pdf is a .pdf file with a step-by-step description of how to do this.
In later versions of IAR Embedded Workbench for ARM, more support for this has been added, and this topic is covered in Help > C-SPY Debugging Guide, chapter "Getting started using event logging".
All product names are trademarks or registered trademarks of their respective owners.