Nevertheless, the distinction between a "realtime" and "offline" timestretch isn't simple, as the stretch algorithms usually thought of as being "realtime" aren't (of course)
truly realtime in the strict sense, otherwise they would be able to see into the future when speeding up material

. However, they are functionally realtime enough to allow smooth parameter changes. Similarly, the "offline" timestretches often complete their task in considerably less time than the length of the material they're stretching, and to some extent they can be integrated into a sequencer/multitracker to appear to function in "realtime" on a given clip, too.
The point is, if a timestretch algorithm is implemented in an offline manner (throwing a progress dialog and making you wait for the transformation to be over), it isn't by all means a guarantee that it would produce a better result than a perceived "realtime" one.
That being said, I'd be very interested to know what offline timestretchers are there nowadays that use considerably heavier calculations than the algos in "realtime" applications? How much better do they sound, and are there demos available?