I have a similar situation. We have over 100 table definitions in the dataset. When using WinXP, 5 out of 9 times VS would crash during saving due to OutOfMemoryException. Updated the OS to Windows 7 (32) seems to speed up the saving and the designer performance. However it still occasionally cause OutOfMemoryException in Visual Studio during saving. I usually end up restarting VS, open just the dataset designer, and then save the dataset.
I have a Duo Core with 4 GB of ram. It takes about 1 to 2 minutes to save the Dataset. Not as slow as 10 to 15 min, but it is still slow. During the saving, the designer regenerate the dataset.design.cs file. My designer file is about 9 MB. You may want to check the size of your designer file and gauges if your VS saving that a long time.
My suggestions are:
1. If you have ReSharper installed, try disable it in case ReSharper analysis the generated designer file.
2. Delete the generated designer file, and see if generating a new one is faster.
3. Not sure if it matters. Check how complicated your table definition is. It may be possible if you have some kind of circular table relationships that slow down the designer file generation.