The intellisense dropdown uses the standard ListBox which calls SendMessage to the native listbox instance once for each new intellisense item. For many items, this can be incredibly slow.

However, it seems that listbox supports pre-allocation for memory when a large number of items will be added:https://docs.microsoft.com/en-us/windows/win32/controls/lb-initstorage

Making direct use of this API would enable performance improvements when a large number of intellisense items (up to 32k) are present - on my machine (32 core), a few thousand results in 10-30 seconds of delay every time a new intellisense dropdown opens, which is almost completely unusable. This fix could potentially make such scenarios much more accessible, even for low numbers of cores or low power machines such as laptops.