iTunes now takes up 100% of CPU since updating to 11.3
Since upgrading to iTunes 11.3 I have experienced a very similar problem.  Every time I refresh my podcasts, iTunes' memory/CPU usage surges, and the iTunes UI freezes up for 5-10 minutes.

I haven't found a complete solution, but I have stumbled across a temporary workaround.  On my system, the problem only seems to occur when I refresh podcasts while looking at them in the List view, as in the screenshot below:



If instead I refresh podcasts from the All Unplayed view, there is no freezing at all:



It appears that the issue is related to viewing the podcast list while refreshing all podcasts.  (You can even avoid the issue if you click refresh in List view and then quickly click away to another view, like Music.)

Not a super satisfying answer, I know, but for me it has returned iTunes to a semi-usable state.