There is theoretical inefficiency and practical inefficiency. Something might be O(n^3).. but if your n is small (as in backblaze case, where you do it on a file by file basis, rather than for your filesystem), it is still useful.

In other cases, your optimal algorithm might have a large constant cost (setup cost etc) which for small n might make it practically inefficient. n^2+c1 and n^3 + c2, but c2 >>> c1 happens a lot.