VRAM
The memory soldered to a graphics card, separate from the computer's system RAM. It is the hard ceiling for local models on a PC: consumer NVIDIA cards ship with 8 to 24 GB, and the high end reaches 32 GB. A model that does not fit spills into system RAM and slows down by an order of magnitude.
The two platforms fail differently, and it is worth knowing which failure you are shopping against. NVIDIA hits a wall: under the limit it is very fast, over it the model barely runs. Apple Silicon degrades instead, because unified memory lets a model spread into a much larger pool at a bandwidth that drops off gradually.
Which is why the same money buys speed on one platform and capacity on the other.