Very comprehensive ISO tests. With a small 1/2.5" image sensor, the noise is unsurprisingly high at ISO 400 and above. The difference of it over the Panasonic cameras is that it does not apply as aggressive noise reduction which take away details.
I think without IS, most people should be able to use as low as about 1/100 without obvious handshake blur at max. zoom (84.2mm actual focal length, about 500mm on 35mm format equivalent) on this SP-550 UZ.
This is true for other long-zoom prosumers with a max. actual focal length of 70-88mm and should be about the same here despite many people still insisting on using the 1/35mm format equivalent focal length (1/500 in this case) as a guideline without verifying with the real world.
Slightly faster than 1/actual focal length is sufficient for most people without very shaky hands to avoid handshake blur. Hence, here we are looking at slightly faster than 1/85 being sufficiently fast enough to avoid obvious handshake blur for most people.
With IS, it should be about 2-3 stops slower shutter speed than wiithout IS for compact cameras. For most people, it's 2 stops. So most people should be able to shoot at 2 stops slower shutter speed than 1/85 at max. zoom, meaning about 1/20 with IS for the SP-550 UZ at 84.2mm actual focal length (500mm on 35mm format equivalent focal length). The threshold for most people with a comparable Panasonic prosumer camera with an actual focal length of 88mm is also about there, 1/25 with OIS at the long end.
If the camera is not zoomed at all, the actual focal length is only about 4.7mm (28mm on 35mm format equivalent) and you should be able to avoid handshake blur even with shutter speed at 1/10 without IS.
However, motion blur is usually a problem when shutter speed goes 1/30 or slower. So a higher ISO is needed to push up the shutter speed to avoid motion blur. Apparently, noise is quite bad at ISO 400 and above for this SP-550 UZ which has a very small 1/2.5" sensor (which makes the magnification of the focal length 6x to the 35mm format equivalent).