Didn't keep Phelps from winning the 200 fly and setting a record. His googles were leaking the entire race.
Leaking I could always deal with. Even if one lense completely filled I was Ok. But when they both fill you have a serious handicap. The worst is when they momentarily partially flip down when you hit the water off the blocks. Both lenses fill completely, and then reseal over your eyes. This effectively blinds you to seeing much beyond light and dark. Your ability to track straight down the lane is impaired, as is timing your turns. Not conducive to a good time...
Way back when I swam, our pools were designed as described above. The depth was less, which makes a big difference, and we only had 8 lanes, which also makes a big difference. Our lane dividers were designed with the same goals in mind, as were the water-level gutters. I'm sure that improvements have been made over the last 20 years, but the intention has always been the same.
I know we could feel the difference when we hit pools that were deeper or shallower than ours, and could see it in our times. I swam in the normal "Speedo" days, and 20 years later I'm still amazed at how "draggy" a pair of regular trunks feel.
Back then, we shaved down for state level meets, or for conference or sectionals if it was doubtful you'd make the state cuts. Nairing wasn't considered good enough, as removing the rough, dead surface skin layers was the goal as much as removing the hair. We also used lubricating gell of some sort (don't remember the name). It wasn't as effective as the modern methods I'm sure, but it was good enough that you didn't dare get it on areas of your body where you WANTED drag (palms, feet, inner arms, etc...) I never used a cap, but shaved my head. The cap was considered "faster" though.
MtnMan