Efficient, Readable Refactoring

This PHP code at ABOUT YOU applies category discount rules across the product catalog and looks products up by ID for order processing. It works on small test catalogs, but a teammate flagged that discount totals look inconsistent between runs, and that catalog-wide jobs using it are much slower than they should be.

function applyDiscounts($products, $discountRules) {
    $result = [];
    foreach ($products as $product) {
        $finalPrice = $product['price'];
        foreach ($discountRules as $rule) {
            if ($rule['category'] == $product['category']) {
                $finalPrice = $finalPrice - ($finalPrice * $rule['percent'] / 100);
            }
        }
        $product['finalPrice'] = $finalPrice;
        array_push($result, $product);
    }
    return $result;
}

function getProductById($products, $id) {
    foreach ($products as $product) {
        if ($product['id'] == $id) {
            return $product;
        }
    }
    return null;
}

Refactor this to be more efficient while keeping it readable for the rest of the team. Point out both the performance problems and anything that could explain the inconsistent discount totals.