Well, there's a number of things you can do to increase speed. You can use adaptive sampling, which can be especially handy with things such as indirect lighting and soft shadows or blurry reflections and refractions and such.

Also you can use quasi-random sampling (like for instance monte-carlo raytracing) to reduce artefacts near corners which should make it possible to render with less samples.

And ofcourse there's always shortcuts such as scanline rendering yea. Also you could use hybrid systems that use scanline technology where possible and only introduce raytracing when neccesary.

Things such as TA could be done with more efficient algorithms such as Final Gathering or GI. These algorithms allow for use of samples in scene space rather then in pixels, so you can blur them in scene space and reduce noise. High sampling rates is often only needed in small corners, on larger surfaces larger blurred samples are just fine.

I'm sure there's more that can be done but that goes beyond my knowledge of mathematics and computer graphics technology.

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