There should be a simple autocomplete(intellisense) in idle.

If I understand the issue correctly, such a feature already exists. Currently, the wait before showing the list of completions is 2 seconds (https://github.com/python/cpython/blob/master/Lib/idlelib/config-extensions.def#L7) though this can be changed in settings. It is not as advanced as you would find in other IDEs, though it generally does the job.

IDLE already has autocomplete of names, attributes, and string paths.  This is documented in the Completion subsection of the Editing and Navigation section of the doc, easily accessible on the Help menu.  Please read the doc before suggesting enhancements.

Issue 27609 summarizes approximately 10 existing bpo issues for improving completions.  Please check the list there before suggesting a specific change.

I believe that 'Intellisense' is a Microsoft's trademark for a Visual Studio feature.  It should not be used generically.  Since I only ever used VS, in the past, to compile python.exe, I never knew what Intellisense did, just that it made me wait when I opened VS.  (/PCBuild in the CPython repository now has build.bat to directly invoke the compiler without this delay.)

I am guessing from your wording that at least part of the Intellisense delay is to recompile the database needed for completions.  As a C++ program, VS cannot import python the current text of python modules the way that a python-coded IDE can.  It might be nice to add something to the stdlib that could be used by any python editor or shell, but I will not pursue that until existing issues are finished.