There is no doubt that hypohydration negatively affects performance, both independently of thirst and independently of knowledge of our hydration status. Recent blind studies (here, here, here, and here) confirm the results of earlier research, whilst addressing its proposed methodological issues. There has long been, and remains, considerable question about the precise mechanics behind the loss of performance observed, with cardiovascular, metabolic, neuromuscular, and temperature regulatory factors being considered. The latter remains the prominent theory due to our knowledge of the relationship between core temperature and muscular exhaustion, but it is likely a combination of all of those factors.
We begin to feel thirst only once our dehydration has reached around 2% of our body mass. By that time, the data suggest that we have already lost approximately 10% potential power output. And by the time we have lost 5% of our body mass, our performance has suffered by more than 30%! There exists variation between the differences of mean and peak power output, as well as between different studies. However, the trend is clear. It is recommended, therefore, that hydration be programmed or otherwise done ad libitum to match fluid losses. And this has become standard practice amongst elite athletes in all endurance sports.
Rates of dehydration vary enormously based upon the individual, their physical output, and environmental conditions. One of the easiest and most practical ways of monitoring this is by calculating our own sweat rates during different exercise bouts. (These rates will vary with the type of exercise we are performing, typically being the greatest with those in which we are most proficient.) This is done by taking our scale mass immediately before and after the bout of exercise, and monitoring our total fluid intake during the course of the bout.
Sweat rate (l) = ( pre-exericse mass (kg) + fluid intake during bout (l) − post-exercise mass (kg) ) × duration of exercise (min) / 60
Typical values range between about 0.5-3.0 litres per hour. Ideally, we would replace our fluid loss with at least the same quantity being lost, gradually and at intervals during the course of the bout.
I hope that is clear and helpful.