A shared web hosting service is an agreement whereby each user gets a certain portion of total available resources.
Multiple domains are hosted by the same server, which means that the server’s capabilities are split between multiple domains.
This means that these websites will not experience as robust of performance as a website that has a dedicated server applying all of its resources solely to one website.
If requests come in for multiple websites on the same server simultaneously, the server will only be able to transfer so much information at a single time.
For low-bandwidth websites, the delays will often be unnoticeable, but for more burdensome websites, the delay can be significant.