Speed up looping through slicers and .selected true if slicer item name matches input words
Invariants, variables which don't change in a loop, should be set before entering the loop. Additionally, if you want to store the value of a Boolean expression, just store the expression's return value:  

Sub SearchSlicer()
    Dim oSc As SlicerCache
    Dim oSi As SlicerItem
    Dim searchword As Variant

    searchword = InputBox("Enter a searchword")
    searchword = "*" &amp; UCase(searchword) &amp; "*"
    Set oSc = ThisWorkbook.SlicerCaches("Slicer_SEARCH_WORDS")

    For Each oSi In oSc.SlicerItems
        oSi.Selected = oSi.Name Like searchword
    Next oSi
End Sub


But I doubt this will speed up the loop much. Most likely the 'like' operator is where most of the time is spent (and I don't see any alternative to it).