Why does loadMultiple load 1000 nodes 9.2 times faster than loading each node one by one?
It's likely that when you are loading individual nodes, you are loading the entity type manager, then for each node retrieve the storage, do a query to get the node data, and build the node. With the multiple process, some of the DB queries are likely combined, and the processing will be in a batch rather than having to run through the code multiple times.