Spybot-S&D 1.6 already improved the scan speed a lot, still, people complain about the time a full system scan takes again. This graph shows a "nice" explanation: the number of detection patterns used to detect malware has doubled since the release of 1.6 in July this year, to 343,003 detection patterns in this weeks update.
The graphs first steep increase can be explained with the addition of detections that needed the new capabilities of the 1.6 engine, and should you wonder about the following valley, that was a point where the new capabilities were used to reduce the amount of patterns needed, without actually reducing the detection range.
Conclusion? We have a massively improved detection range, and we're going to improve the scanning time this costs in 2.0 to counter the negative side (speed) of it.