What exactly are you doing? Shrink or growth?

The latter should be instant.

The former can take a lot of time depending on how little slack you leave and how fragmented your free space is.

It has to move all the data behind the given offset in front of it.
The smaller the target size, the more data is has to move and the fewer space it has to put it.

If it has to "stuff" the data into a bunch of mostly full fragmented chunks, that can be very slow.

What does `iomon` say on drive activity?