loops are the obvious performance bottlenecks we see in applications every day.
sometimes loops are slower then necessary, because method/function-calls which do not depend on a variable, which changes with iterations is used within the loop, but could just be moved outside the loop.

pseudo-code:

$x = 3;
foreach($iterator as $value) {
 $this->somePureMethod($x);
 // more loop body
}

since the method is pure, invoking it on every iteration is unnecessary and the code could be rewritten to

$x = 3;
this->somePureMethod($x);
foreach($iterator as $value) {
 // loop body
}

should phpstan error when pure functions/methods are used within a loop, but do not depend on its iterations?

another benefit of moving code outside a loop, is more readable code