Redis is single-threaded, so DEL on a structure with a million members blocks every other client for as long as freeing it takes — which is milliseconds for a string and hundreds of milliseconds for a large hash.
DEL session:index # frees synchronously, blocks the server
UNLINK session:index # removes the key now, frees in a background thread
# and the same choice for the whole database
FLUSHALL ASYNCThe key is unreachable immediately either way; only the memory reclamation moves. The cost is that peak memory is higher, because the freeing lags. There is a lazyfree-lazy-user-del setting that makes DEL behave as UNLINK globally, which is the right default for most workloads and is off by default for compatibility.