Swim Workout Frequency and Swim Times

OK, it s a no brainer (generally -- I accept that overtraining, reinforcing bad technique, etc. are exceptions to the rule) that the more you practice the faster you'll get, but I've been surprised at how little extra training it takes to make a big impact.  I've been nursing a sore hip running and, as a result, have found more time to swim.  I'm doing more or less the same things in each workout, but have gone from, on average 1.5-2.0 swims per week to 2.5 swims per week - more or less an extra workout here and there.  Exact times are not important and are relative for each person, but I've seen a significant drop in my 500 and 1,000 times and the ease with which I can hold a faster pace for 100 intervals.

I just wanted to chime in that even modest increases in frequency (one extra swim every week or two) really seem to have a big impact.   Anyone have experience to share on how easy it is to maintain those gains once they are achieved - do they dissappear or do you hang on to them if you ease back the frequency a little bit?