Btrees don't waste much memory, while hash tables have to have excess capacity if you want them to go fast.

That's true for on-disk b-trees which typically have large node sizes (typically 4KB), but in-memory btrees tend to aim for CPU cache lines (typically a small multiple of 32B), and thus do actually waste a fair amount of memory with their comparatively low branching factor, and thus relatively large number of branches compared to their number of leaves.

The waste of abseil's b-tree, for example, is small per value: https://abseil.io/about/design/btree

The efficiency compared to hash tables very much does carry over to the small b-trees used for in-memory data.