I actually noticed only a slight difference with the cap on. I swam a mile and had to actually concentrate to notice any difference. Maybe my breathing is less intense than others when I swim.

The Cardio Cap is a great addition to my Swimmers Snorkel.  Reduces my air intake and makes me breath harder during workout.  When i swim without it I notice a big difference.  I use to take a breath every stroke now i take a breath every other stroke.  Im not giving all the credit to the Cardio Cap but it certainley helps.

Good snorkel, although I am not comfortable without nose plugs.  Sometimes wish it curved back a little more as if I am looking straight down at the bottom of the pool and "swim downhill" or "press the buoy", I can start getting water in it.  But 95% of the time it is fine.  Maybe should have gone with the free style snorkel.

Easy to install. I am beginning to tell a difference. The cap also helps keep water out.

This cap does make me work a little harder taking deep breaths.  After a couple weeks, I'm not challenged at all.  It would be great if there were different models/versions that required increasing levels of effort.