AfterI opening a Prism file, all analyses and curve fits are recalculated, which can be slow.

Prism lets you save the file in two formats. 

PZFX

The pzfx format never contains analysis results. So when you open file, the results need to be recalculated. This doesn't happen all at once, but rather happens when you view results pages or graphs that include results. 

For curve fitting, the slowest part (most of the time) is computing the asymmetrical likelihood ratio confidence intervals (new in Prism 7). If you switch to symmetrical confidence intervals, the calculations will go much faster. 

The other calculation that can be slow are some nonparametric tests. 

PZF

PZF files can contain results or not. Choose in the File and Printer tab of Preferenes. 

Choosing PZFX or PZF

When you save a file, you can choose the format at the bottom of the Save dialog. You can change to the other format later using File..Save as. 

You can change the default format in the File & Printer tab of Preferences.