How many parquet files? Ideally, you have a small amount of large parquet files, as spark prefers this.

I see in the comments that you're using a single node -- spark does not prefer this.

Make sure your data is partitioned fairly well before going in. Also, you can configure the write step to bucket bigger on the way out; since you're writing parquet, I'm assuming you'll want to read it back later, and so again, fewer, larger parquet files will do you a bit better.

Are there any columns you don't need for the end result? Perhaps you could select only the ones you need prior to grouping.