It turns out that water is not very viscous, so I'd ignore that factor. For example, a quick search indicates that milk is three times as viscous as water and while I doubt many people have tested this hypothesis, I imagine swimming through milk is not perceptibly more difficult than swimming through water. Furthermore, higher viscosity should help with your propulsion about as much as it would hinder you with drag. The effects should roughly cancel out and it should not be noticeably more difficult to swim through a somewhat more or less viscous substance.
As for density, I'm also skeptical of that as a standalone cause. The Dead Sea (saturated saltwater) is 24 percent more dense than pure water. Not only is this pool water likely not saturated, we would expect it to be easier to swim in more dense water, so we need to imagine some solute that decreases the density of the water. I didn't have much luck searching for such a substance (ethanol gets frequent mention, but there shouldn't be much ethanol in this pool water). Dissolved gas comes to mind, but since you said this pool is at a high altitude, there should be less gas in the water, not more. The density is directly affected by the ambient pressure and temperature, but
another quick search indicates that it's affected by
maybe one percent if you're talking about warm water at low pressure versus cold water at high pressure.
So in short, I'm doubling down on altitude being the chief factor here. I think that you experienced some mild altitude sickness that you were able to overlook in smaller pools at higher altitudes.
