Xbox Series X Memory Card. The external memory card plugged into the console and allowed players to save game data directly to the memory card. Seagate's Xbox Series X Storage Expansion Card technology delivers additional game storage at peak speeds, replicating the console's internal SSD experience.
So even the "slow" memory pool on Xbox Series X is faster than the memory available to developers on Xbox One X.
This is because of how versatile it was designed and how much flexibility is given to console.
The Seagate Storage Expansion Card is a proprietary solution akin to memory cards of past, ensuring speeds that deliver high performance. The external memory card plugged into the console and allowed players to save game data directly to the memory card. Traditionally, memory cards have been plugged into the front of gaming consoles, but the Series X puts the port for the Seagate Storage Expansion Card on the back.