Generating HIE files eats up a lot of memory and takes a long time. There are at least two big parts to this:

• Every expression in the AST is desugared via deSugarExpr for the purpose of extracting a type. GHC ends up doing the same work multiple times for the same nodes. Ex: in order to desugar HsApp _ fun arg, GHC desugars both fun and arg, yet the HIE code doesn't cache that (so it ends up desugaring fun and arg at least once more).
• There is a lot of stuff being written into these HIE files, making them quite big (even after compressing/deduplicating type information). One data point: the largest HIE file at time of writing is Parser.hie, which is 7.2MB, compared to just 58K for Parser.hiQ Writing large files is naturally going to take up memory and be slow.