Jag tror att alla dessa korrelerade underfrågor dödar dig. Prova detta:
SELECT p2c.pid AS productNumber,
p.name AS productName,
COUNT(*) AS registered,
SUM(date_add(from_unixtime(purchased), INTERVAL 5 YEAR) >= CURDATE()) AS inWarranty,
SUM(date_add(from_unixtime(purchased), INTERVAL 5 YEAR) < CURDATE()) AS outOfWarranty,
DATE_FORMAT( MAX( from_unixtime(purchased) ), '%d.%m.%Y') AS lastPurchased,
DATE_FORMAT( date_add( MAX( from_unixtime(purchased) ), INTERVAL 5 YEAR), '%d.%m.%Y') AS warrantyUntil
FROM products2customers p2c
JOIN products p ON p.id = p2c.pid
GROUP BY p2c.pid
ORDER BY inWarranty DESC