Filtering options are constrained by what's available on the GPU and the result depends in part on your graphic card. The result is not that bad if the card supports multisampling. Also, FastPictureViewer is is optimized for speed and there is no faster zooming-in and zooming-out than the instantaneous performance offered by FPV in GPU-accelerated mode.
Some applications offers different filtering options, some providing better results in general, but are also considerably slower with those options enabled.
Below is a screenshot showing a
highly reduced 24MP image (from a Sony A900) and I fail to see any unacceptable jaggies that makes the displayed result unsuitable for its intended purpose but, that said, be assured that I keep an eye on what's possible in GPU-accelerated viewing and I'd not mind offering even better results when possible

The card used below was a GeForce GTX 460 with 1GB, not entry-level but not high-end either, which supports multisampling.

A portion of the same image reduced to 1200 vertical pixels:

A tiny portion of the original 24MP image:
