I haven't formally tested but I do notice on windier days (lake swim) it makes a bigger difference, probably 5-8 seconds per 100m but I'm a slow swimmer to begin with. I have one of those bigger ones where it acts as a dry sack.

Similar. I did a solo swim with my wetsuit a few weeks ago and was 8 seconds per 100 slower than the day before (day before was twice the distance too). Didn't think that could be all fatigue. I've only swam without the wetsuit since. Typically in a rush to get out of there so haven't taken the time to do a short test.