As checkout page includes customer information, product data, price rules information.
1. We should check is there any observer linked to those entities and running unnecessarily. This happens with most of the extensions and custom built modules.
2. As a next step check how many price rules you have. Removing expired rules is suggested.
3. Merging css/js is always suggestible.
4. Optimize your PHP code as it plays a key role in performance. Especially if there are any improperly written loops. This will become a bottleneck situation. Follow the standards and keep your code clean.