Example embodiments relate to classifying application protocol interfaces (APIs) in terms of their relation to user experience. Example embodiments may determine that a user action occurred and monitor a first thread separate from a user interface (UI) thread of the computing device. The first thread may process a first API in response to the occurrence of the user action. A determination may be made as to whether the first API being processed on the first thread is related to the user action. Responsive to the first API being related to the user action, a determination may be made as to whether the first API causes the first thread to update the UI thread of the computing device. Responsive to the UI thread being updated, the first API may be tagged as affecting user experience.