I would suggest that the task loading in cold water is greater. Warm water, light equipment, good visibility. you will be narked, but it will be less obvious to you as you are not actually having to *think*.
Now, in the darker colder water, you have different equipment, (Dry suit?) darker water, with much poorer vis. At this point you actually have to start thinking about things that you didn't in clear tropical water. The more you think, the more you realise you can't think. The more you realise you can't think the more narked you *feel*.
Narcosis is how your brain experiences what is going on around you under the influence of nitrogen. Don't have to do much thinking, and you don't *feel* as narked, even though you are.
Next time you are down in clear tropical water get your buddy to give you a set of tables and a problem to work out. Time how long it takes. Something that will take you 30 seconds on the surface will take 2 - 3 mins at depth (30m -99ft is good enough), even thought you don't realise it is taking that long. Repeat in cold murkey water at same depth. - The times should be the roughly the same, as you have the same ammount of nitrogen in your body.
Just .02
Jon T