A lot — Redis is single threaded, so KEYS blocks every other client for the whole scan. At 8 million keys that is comfortably into multi-second territory, and everything else queues behind it.

Use SCAN, which returns a cursor and does bounded work per call:

1let cursor = "0"
2do {
3  const [next, keys] = await redis.scan(cursor, "MATCH", "session:*", "COUNT", 500)
4  cursor = next
5  if (keys.length) await redis.unlink(...keys)
6} while (cursor !== "0")

1let cursor = "0"
2do {
3  const [next, keys] = await redis.scan(cursor, "MATCH", "session:*", "COUNT", 500)
4  cursor = next
5  if (keys.length) await redis.unlink(...keys)
6} while (cursor !== "0")

Note UNLINK rather than DEL — it frees memory on a background thread instead of blocking.

Better still, don't scan at all: set a TTL when you create the session and let Redis expire them for you.