Description
Runtime cache on the serailizers / normalizers component system.

Example
We are using symfony/serializer as a dependency in Drupal. There is this module that seems to abuse this piece of code. and is causing very big delays (like 10-20% or request time) just finding the right normalizers.

Furhter details can be seen in Drupal's issue queue for jsonapi module: here

Direction
I've managed to drop overhead like 15-20 times with the following pseudo code.
I know it's not production ready but for you can get the idea:

Symfony\Component\Serializer\Serializer::getNormalizer($data, $format, array $context)

/**
     * Returns a matching normalizer.
     *
     * @param mixed  $data    Data to get the serializer for
     * @param string $format  Format name, present to give the option to normalizers to act differently based on formats
     * @param array  $context Options available to the normalizer
     *
     * @return NormalizerInterface|null
     */
    private function getNormalizer($data, $format, array $context)
    {
        static $cache = [];

        $cid = get_class($data) . ':' . $format;
        if (isset($cache[$cid])) {
          return $cache[$cid];
        }

        foreach ($this->normalizers as $normalizer) {
            if ($normalizer instanceof NormalizerInterface && $normalizer->supportsNormalization($data, $format, $context)) {
                return $cache[$cid] = $normalizer;
            }
        }
    }

Additional thoughts

• This can be moved to an internal property to be managed along-side the list of normalizers. Any time a normalizer list is mutated - reinitialize the property.
• At the moment this is not handling native types well (ints, strings) likely this overhead can be further reduced.
• Maybe just expose the getting of normalizers from private to protected, so this kind of optimizations can be implemented outside from here.