$reservedChars = null === $allowChars 
    ? self::RESERVED_CHARACTERS
    : str_replace(str_split($allowChars), '', self::RESERVED_CHARACTERS);

<?php

class CacheItem {
    private const RESERVED_CHARACTERS = '{}()/\@:';

    /**
     * Validates a cache key according to PSR-6.
     *
     * @param mixed $key The key to validate
     *
     * @throws InvalidArgumentException When $key is not valid
     */
    public static function validateKey($key, string $allowChars = null): string
    {
        if (!\is_string($key)) {
            throw new InvalidArgumentException(sprintf('Cache key must be string, "%s" given.', get_debug_type($key)));
        }
        if ('' === $key) {
            throw new InvalidArgumentException('Cache key length must be greater than zero.');
        }
        $reservedChars = null === $allowChars ? self::RESERVED_CHARACTERS : str_replace(str_split($allowChars), '', self::RESERVED_CHARACTERS);
        if ('' !== $reservedChars && false !== strpbrk($key, $reservedChars)) {
            throw new InvalidArgumentException(sprintf('Cache key "%s" contains reserved characters "%s".', $key, $reservedChars));
        }

        return $key;
    }

    /**
     * Validates a cache key according to PSR-6.
     *
     * @param mixed $key The key to validate
     *
     * @throws InvalidArgumentException When $key is not valid
     */
    public static function validateKeyByOverride($key, string $reservedChars = self::RESERVED_CHARACTERS): string
    {
        if (!\is_string($key)) {
            throw new InvalidArgumentException(sprintf('Cache key must be string, "%s" given.', get_debug_type($key)));
        }
        if ('' === $key) {
            throw new InvalidArgumentException('Cache key length must be greater than zero.');
        }
        if ('' !== $reservedChars && false !== strpbrk($key, $reservedChars)) {
            throw new InvalidArgumentException(sprintf('Cache key "%s" contains reserved characters "%s".', $key, $reservedChars));
        }

        return $key;
    }
}

$key = "product:123456:discount";
$iterations = 1000000;

// Testing the original approach
$start_time = microtime(true);
for ($i = 0; $i < $iterations; $i++) {
    CacheItem::validateKey($key, ':');
}
$end_time = microtime(true);
$time_original = $end_time - $start_time;
echo "Original Method Time: " . $time_original . " seconds\n";

// Testing the optimized approach
$start_time = microtime(true);
for ($i = 0; $i < $iterations; $i++) {
    CacheItem::validateKeyByOverride($key, '{}()/\@X');
}
$end_time = microtime(true);
$time_optimized = $end_time - $start_time;
echo "Optimized Method Time: " . $time_optimized . " seconds\n";