Like most lightweight something to 300mm zooms, this lens starts out strongly at it’s short end, but performance falls off towards 300mm. Having said that, the extent of the drop in resolution is less with this lens than with most.
Sharpness in the center of the frame at 100mm is simply excellent - even in the corners at full aperture there is very little fall in resolution. By 200mm the center is still excellent, but there is more noticeable softness at the edges, and by 300mm central resolution is less crisp but still very good. Even right in the corner of the frame at 300mm and full aperture is just about OK. At all focal lengths stopping down to f8 or 11 brings a steady improvement but results are still best at 100mm.
Focusing - thanks to Canon’s USM - is fast and very accurate.