Why are Spark Datagrid headers sorting my data so slowly in Air?

I wrote a AIR program two different ways, one using a Spark Datagrid and another using the older MX Datagrid. I currently have 40k records displaying in the datagrid and the end user is going to want to sort the data by clicking on the table headers. When I used the older MX version, the sorts happened fairly quickly, I was really surprised. But the Spark version is so incredibly slow, sometimes taking up to a minute to sort the data. I am not doing anything really complicated with item renderers. I have one that just displays a tiny picture icon, and another that formats an integer into money using a currency Formatter. I ended up needing to use the Spark Datagrid for presentation purposes. Any idea why the sorting is happening so slowly, and are you aware of any workarounds to make the sorting on the headers faster? Thanks!