I have never heard of "too" slow. Computers can malfunction - that is true - however if all it wanted was you to slow up why not slow up? If you ascend too fast the computer would penalise you on the next dive.
I always make a point to come up slowly. That said, I still find a way to set off the ascent rate alarm. The settings are just too conservative.
How slow?
Research supports 30 FPM as a generally accepted safe rate. Most computers use that as the planned ascent rate, as do most decompression software programs. As people have noted earlier in the thread, a fast ascent warning in your computer log is meaningless. It could have happened that you exceeded that rate for a few seconds during an otherwise very slow ascent.
Yes, there is such a thing as too slow. The rate you ascend has to be a balance between off-gassing tissues safely while at the same time the slower tissues are still on-gassing. Ascend too slowly and you add to your decompression obligations more than you benefit from the off-gassing of the faster tissues.
This past weekend I was instructing a bunch of students in several different classes. It included some DM candidates, whom I had lead the ascents on several dives. The first time we did it, the ascent from about 70 feet to the safety stop took nearly 5 minutes, an ascent rate of about 12 FPM. That is about a third of what is usually considered the ideal rate. We talked about it, and on the next dive the ascent time was reduced to about 4 minutes--roughly 15 FPM. They eventually were able to force themselves to bring us up at what seemed to them to be a screamingly fast 20 FPM.
A few years ago two of my friends got bent on a decompression dive for which they were using computers in gauge mode, having been taught to use "the computers between their ears" instead of computer algorithms. They began their ascent according to plan. When they reached their first deep stop, which should have taken less than a minute, they began to follow their plan of a specific number of minutes at each of the following stops. They did not notice that their ascent to that first stop had taken them about 4 minutes--much, much slower than their plan called for. They discovered that when they downloaded the dive data from the computers that were in gauge mode. They were trying to figure out why they got bent on that dive.
A few months ago I was with a group of divers doing decompression dives off of a boat in Florida. Each of the groups had roughly the same dive plans calling for a total run time of about 70 minutes. Everyone but one group was on the boat long before the last group surfaced, a group that had started up the ascent line the same time that my group did. They said they could not understand it--when they started up the line, their computers indicated that they would have the planned 70 minute dive. When they got to their first stop, though, both of their computers had them doing a more than 80 minute dive, with extended decompression stops. I had no trouble understanding it. Even though we started up the line at the same time and had our first stops at the same depth, when I got to that stop they were not in sight. I had completed several stops by the time their slow creep up the line had gotten them to the first stop. While they were ascending far too slowly, their computers were correctly interpreting the ascent as continued bottom time and adding decompression time to the coming stops.
I firmly believe that a lot of people understand that they are supposed to do a slow ascent, but they take that warning far too far.
