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You want to use pressure rated fittings, Schedule 40 or better, for all pool plumbing.
The difference between a stock 90 and a sweep 90 is a small fraction of a percent. Those fractions of a percent do add up slowly if you replace a large number of 90s, but even so it will only amount to a couple of percent at best. Annoyingly, Schedule 40 sweep 90s tend to be difficult to find. In an ideal world, using sweep 90s is a great idea. But for most people purchasing the better fitting is enough of a hassle that they give up on the small efficiency improvement.
A few states are promoting the use of the better fittings as part of their green energy programs. Hopefully that will result in better availability at some point.