Since CLion tries to parse your whole code base, including all the header files used, to understand your code correctly and provide accurate code completion, code navigation, refactorings and other intelligent features. On large code bases (especially when using Boost), performance and memory issues during indexing and editing are possible.
Increasing memory may help to resolve performance problems. To check the memory usage, enable the memory indicator in Preferences/Settings | Appearance & Behavior | Appearance | Show memory indicator.
To report a performance problem, please follow these instructions. To find IDE logs to attach to the problem, use this link.