I wouldn't be too worried about 15 minutes, we've had several with scans taking hours to days with no apparent explanation. Also, both FireFox and SpywareBlaster have effects on the scan time for Spybot S&D. This is because Spybot must check the additional FireFox folders for both cookies, Cache files and other items that don't normally exist with just IE.
With Spyware Blaster, the thousands of entries it creates in the registry must be searched through for those that might have been created by spyware instead. It's often stated that SpywareBlaster doesn't create any overhead, but that's really not true as you can see from this, but it is worth the overhead for the protection it gives.
Personally, I'd say you're suffering from anti-malware overload. We're seeing more of this lately as people try to run everything in the book to insure their safety. Other then taking lots of time, it also starts to have the issues you've been seeing. You should never have more then 1 active (real-time) antivirus and 1 active antispyware running at once. Watch out, some of the antivirus are now adding antispyware, so you might be doing this and not even know it.
The reason for this is that when one of the programs sees a 'bad' change, and tries to block or fix it, the another may see it too and they both try to fix it at once, causing a conflict. The likely reason that Spybot's scans are being slowed is that one or more of your real-time programs is checking each access made by Spybot to determine if it might be spyware, causing much more overhead then Spybot would itself.
This is why I recommended running a Spybot scan in Safe Mode, since most other software isn't loaded in this mode. That way you can see how Spybot runs without all the other programs, without needing to uninstall them all.
It's not a problem to have dozens of scanning programs on the same PC, as long as you only scan with one at a time. It's the multiple real-time programs that really cause the problem. I'd ask around about your favorites and pick either one of each (AV and AS), or one that covers both. You can leave the others installed as backup if you're sure you can turn them off entirely, so they just sit there waiting to scan or be turned on.
So you know, Spybot S&D 1.4 itself has increased significantly in scan time since it's release, in large part due to the addition of Services and many more spyware detections. However, the combination of all the issues mentioned above is making it much worse, and often multiplies rather then adds to the time.