From MICROSOFT website , get SYSINTERNALS tool package.
They have AUTORUNS64 in there.
Run it and post list of stuff that starts on booting up.
You may have some crap like DRM, anticheat, anti virus, or just plain virus. That does some crap every 5sec, and unreal is quite sensitive to harddrive speed or memory usage.
However before you do all that sysinternals stuff, open task manager and see if any process takes over more ram or CPU when unreal has lag.