Three feet per second is 2 mph. That is about 1/2 the max speed of an average free diver. Some trained athletes claim 5 mph for short bursts. Any way you look at it, anything more than 1 mph is not "relaxed". Hawaiians with whom I have dived, descend at what looks to be 2-3 mph. I do the same. Game will continue to move laterally and if the diver slows his pace the fish will be in a different location forcing the diver to swim laterally as well as deeper. However, even when doing reef "inspections" they seem to get down there pretty quick. There should be no oxygen penalty until the dive is over, that is on a two minute dive. Almost anybody can swim for 2 minutes without breathing hard. That is due to the fact that humans have the capacity for "oxygen debt". Feeling out of breath after 20-30 seconds of "hard driving" is either psychological or a sign of something else, something medical. The need to breath is a function of average oxygen uptake and carbon dioxide production. However, the muscles do not need oxygen, they metabolize something called ATP. It is only after ATP depletion, the need to replenish stores, and build up of CO2 are detected that breathing rate, and oxygen uptake, is forcibly increased. The diver normally uses surface intervals to recharge and blow off CO2.