I have seen a few cases recently where people who seem to be following the SLAM process correctly are seeing very slow progress AND the usual complications (leaves on bottom, algae behind light or in ladder rungs) do not seem to be present. Other folks then sometimes post that a "nuclear shock" helped them where a regular SLAM did not -- don't think that's the answer but it seems to me there probably is something else slowing progress.
Based on my own limited experiences SLAM-ing a hot tub (you there in the back, stop snickering) and re-reading a number of threads, I'm starting to suspect that the common problem with these slow SLAMs may be insufficient circulation which results in *local* areas of low FC, allowing algae to grow in these areas even if a suitable FC level is being maintained in the pool as a whole.
In these cases, adding an insane amount of chlorine might help because it would probably raise FC levels everywhere and have a good chance of raising the areas with poor circulation to a sufficiently high FC level to get the algae level trending down rather than up. Of course the same could be said for more-resistant-than-normal algae, or a number of other causes.
At the moment I don't have any great suggestions for how the current practices should be changed if this did turn out to be a common problem -- I don't think nuclear SLAM is the right answer -- but wondering if it's something we should be asking about or experimenting with a bit when SLAMs seem to go on forever in the future.
Just a thought...