Does ios update make iPhone performance better?

no not really, more like a new version full of features not supported by your older model would still be using system memory (not the same as storage) and cpu

which could give a slower experience and possibly less battery time.

Each new update provides improved security for your phone. That alone is a good reason to update your phone as soon as new iOS versions are released As more and more features are added, the speed performance and battery life performance of the phone may decline very slightly. This would be expected.

it depends if it's an update to fix an issue with something being slow

it will make it better

if it's an old iphone then an update full of features for newer devices could slow it down