I suppose it all depends on the order of events. I found last year in the Welsh Open Masters that I couldn't swim the 1500 on the first day and swim more or less my usual program and then finish with the 800. Similarly in a one-day meet last year I found that at the end of the meet I had no stomach for the 800, but usually the longer events come near the beginning, either of the meet or the session. At Gloucester, for example, the first session began with the 400 and the second session with the 200 and all the other races were either 50s or 100s, which give enough time for recuperation - at least the way I swim them. Mind you, I was quite tired when I got to the 50 free, which was the last event. Perhaps if I could learn to sprint it would be different. I remember the first time I swam a LC 50m Back it seemed more strenuous than any other event I had swum in that meet, even the 1500.
Perhaps the way for me to learn to swim faster is to work on the middle distance events. I find I often swim the first 50 of a 100 faster than I do in the actual 50, presumably because I'm trying too hard in the 50 and tying up.