Cache and database
Cache speeds up catalog and content pages, while cart, customer account and payment views need careful handling.
Redis, Memcached, OPcache and MariaDB/MySQL help with larger product catalogs, filters and orders.
Transactional e-mail
SMTP, domain reputation, SPF, DKIM and DMARC matter for order confirmations and customer messages.
SpamBox and daily reports help control retained messages.
Secure shop maintenance
SSL, backups, exploit scanners and updates reduce operational risk after launch.
Technical support helps with integration and performance issues.