There could be lots of reasons for this, depending on what's going on.

Assuming the report is based on a reasonably straightforward query, one of
the worst offenders is where you require Access to keep sections together,
because it may have to go through all the records in a block to discover it
doesn't fit on the page, and then back up to figure out other approaches. It
is possible to really generate a snail by combining this with resizing
sections, hidden sections, and so on.

To see if that is the issue, temporarily set the properties of the sections
(in the Sorting And Grouping box) so it does not have to keep the sections
together.