If you believe default IntelliSense is too slow for UE4 development, try using only the parsers in Visual Assist to populate its IntelliSense-like features.
Update Visual Assist
Make sure you have Visual Assist build 2258 or newer. Support for UE4 improved significantly and parse time greatly shortened in the builds leading to 2258.
Enable Support for UE4
Be sure Visual Assist knows you are working with UE4 via the options dialog if you have build 2258 or newer. If must use an older build, edit the registry as described in the knowledge base article Compatibility with Unreal Engine 4 (UE4).
Create a va_stdafx.h file in the directory of one your .sln or .vcxproj files. Put the following line in the file, and make sure the file ends with a blank line. Replace YOURGAME with the name of your game:
Select Content for Listboxes
Make sure Visual Assist gets content for its enhanced listboxes from its own parsers, via the options dialog of Visual Assist.
Disable Default IntelliSense
Disable default IntelliSense by changing two settings in the options dialog of Visual Studio. (Adjusting these settings eliminate more parsing than what is eliminated when you set Disable IntelliSense to True.)
Tools | Options | Text Editor | C/C++ | Advanced | Disable Database = True
Tools | Options | Text Editor | C/C++ | Advanced | Rescan Solution Interval = 0
Rebuild Symbol Database
Tell Visual Assist to rebuild its symbol database, via its options dialog.
Restart Visual Studio, open your UE4 solution, and wait for parsing to complete.
Visual Assist does not parse source to the depth that a compiler, or default IntelliSense for that matter, parses. Visual Assist parses only to a depth it believes is necessary to provide IntelliSense-like features during edit. The advantage of parsing-only-for-edit is that the IDE remains responsive during edit. The disadvantage is that member and completion listboxes do not always appear, are sometimes not complete, or are not limited to symbols valid only in the current build configuration. (Visual Assist assumes you are editing for all configurations.)