My understanding is that fundamentally the new c4d core has been in c4d for about a decade now, however nothing particularly changed after it was added because of how it was done and the amount of time to truly convert everything over. When the new core was added, c4d was converted to run as a plugin for the new core, so the c4d you see is actually a subset of the new c4d app. As new features were created, they were made to run under the new core. So for example the material node system runs natively in the 'real' c4d whilst xpresso nodes are legacy and thus run in the plugin sandboxed c4d.
Off the top of my head, physics, poly modelling, effectors, volumes, the viewport, timeline, mograph, rendering have all been moved over. With R2024 they sort of moved the main object handling system over which is why some stuff is many times faster. However, some parts are still legacy and hold stuff back. In particular sketch, hair, motion tracking, TP and a few other bits. As these legacy parts continue to be shed, more speedups are likely,