I've worked in government before and understand bureaucracy but there's a certain level of quality even they have standards for. Most government sites aren't massive volume transactions but primarily static brochure type sites that may need some occasional updates making them suitable for many commercial CMSes that are well known and easy to optimize. Adding caching for documents that are almost never updated and adding a CDN layer for the application are inexpensive compared to writing and even hosting the CMS itself (usually on rather expensive TCO on-premise infrastructure). Furthermore, this kind of upgrade helps fulfill use cases of access and availability to government-provided services that is the spirit of the law in many developed countries, especially as citizens increasingly reach to online services for their interaction with governments.
This site is done separately. Your entire application, from initial contact to permanent residency or citizenship, is handled through this website, entirely through submission of electronic forms. While it's basically a form-and-image-and-pdf-and-docx-grabbing site, that's still a step or two above brochure-and-static.
