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millsart it's not so much need for speed, but optimal aperture, which is typically about 2 or 3 stops down from wide open, thats where I like to shoot if I can.
So using that zoom at 50mm, it's already at f6.3 wide open, stopping down to optimal aperture 2 or 3 stops means you have to shoot this lens at f11 or f16! With my Olympus 45mm stopped down to f4 of f5.6 it's already at optimal aperture while this zoom at f6.3 is still wide open rendering a lot poorer performance for landscapes, etc.
Typically with variable aperture zooms I've found they need be stopped down less at the long end. At least for the Panasonic zooms I have (7-14, 14-45, 45-200) there isn't a single one with an optimal aperture past 6.3 at any focal length and it is pretty much impossible to have a optimal aperture past 7.1 or 8 on m43 as diffraction is already limiting the worst lenses available past that point on the 2x crop factor. Never stop a m43 lens down past f/8 unless you do it for DoF and can't focus stack instead.
Like you say though, 6.3 is very slow for 50 and unless it is close to perfect wide open there it isn't very useful.