I have got the same issue. Previewing reports in SSDT2012 takes up to 10x longer as in BIDS2005. I can see the "Reporting Service Preview Processing Host (32bits)" is taking 50+% CPU for very all long time. It makes no difference if data is cached in the “Dataset cache files”.
My experience is that reports with (a lot of) expressions in tablixes are the problem. When you report the same dataset 1 on 1 in a tablix performance seems OK.
I altered some reports with dynamic grouping (grouping based on en expression) because these reports timed out in SSDT! I also removed some CDBL() statements. But our reports still they are dramatically slow.
When I upload a report to Sharepoint (integrated), performance is much better (similar to 2005). So I am thinking about previewing report on our test Site.
ps: most of my reports are based on SSAS Multidimensional v11.0.3393.0 (MDX), on a server with OS2012.
Developing/ testing reports is a very frustrating time consuming job now