DNS (Domain Name System)
That system translates hostnames (or FQDN) into IP addresses so that the request can be routed to the adequate nodes.
DNS will affect SaaS performance in 2 ways:
1. DNS resolution time:
Unless the DNS resolution has been performed and cached recently, translating a hostname into an IP address takes some time (making a request to a DNS server, waiting for the DNS processing to be performed and the response to be transferred back to the client). DNS has a huge impact in case of unavailability, on the other hand the DNS resolution time will affect SaaS performance in a limited way as it will only add delay when a new session is started and the DNS resolution is not found in cache.
According to DNS service providers name servers locations and availability, DNS resolutions are performed based on a geolocation of users and used to redirect them to the closest host for each service. An error in geolocation can strongly affect response times by adding network latency to all interactions with the applications for both the server processing and the data transfer.
2. Network latency from the users to the SaaS platform
The time needed to send packets back and forth between the user and the servers will affect all steps in using the app: establishing a TCP session, setting up a secure TLS connection (see this article for more details on this), making requests to the server, and receiving server responses.
Network latency should be considered not only to the application root servers but also to all hosts involved in the delivery of the application (CDN, authentication servers, API, 3rd party services).