If borrowing the term from Architecture, then a brutalist website should be one that (only) uses the 'raw' (original) elements of design and functionality (cannot separate both in web design), namely: page layout + font face + color palette for the graphic design component, and links + media for the functional (hypermedia) component. As such, I do not see how modernizing the style through functional paradigms, like a flat design UI, breaks the original canon. However, we could argue that anything else, like flashy add-ons such as element transitions, drop-downs, fades, etc, do break the canon and are just a response to trends, just like a bunch of useless zippers and pockets do not add functional value to a trendy jacket but definitely help sell it.
Efficient use of resources and fast loading is a entire different subject. Enter the engineer, leave the architect. It is absolutely possible to program any design paradigm to perform fast and efficiently.
Efficient use of resources and fast loading is a entire different subject. Enter the engineer, leave the architect. It is absolutely possible to program any design paradigm to perform fast and efficiently.