I don't notice it. As an IT person myself It could be a plethora of causes whatever the issue and may not even be their website but your own device or connection that is the problem.
There's so many variables involved it's not as simple as "their system" even if that's how the common folk may see it.
My humble opinion is it's something on your end if it's occurring on multiple websites and happening routinely or regularly. Not all websites are handled by the same tech, equipment, network infrastructure, etc.
Each cruise line would configure, manage, and administer their own (or outsourced to another company to do for them), so the fact it's occurring across different sites for you suggests an issue on your side perhaps with your own Internet connection at the time you experience it. Of course, connections are dynamic and constantly changing depending on resource usage and availability.