The download must have some impact on the frame rate, as especially with the larger image size of the higher res images, the file size will be correspondingly large.
If the 'buffer' can't store enough of these, the usb connection is slow, or there is inadequate RAM or a slow HHD in the PC, then this must impact on the frame rate.
I would say that a usb 1.1 connection is a non starter, but usb 2.0 should be plenty adequate.
I know that the Flea camera, as used by the likes of Damian Peach, runs at something like 120fps, but I don't know at what resolution.
I'm leaning towards the new QHY IMG132 at the moment, as it does offer both Hi and Low Res options at reasonable cost. However, it is very new and as such doesn't have much of a 'track record' as yet.
There is a link to a video review on the camera, located on the Modern Astronomy website, but the review is quite 'low tech', and without much real imaged material.
At the moment, I'm letting Rog do the assessment, as he has just bought one