It saves semi-automatically.
Under Settings, Options, Setup, Numeric Detail Input you can set the Automatic Detail Save frequency, but even if it's disabled, exiting Lacerte normally or viewing Forms will save.
I personally have it set to save every minute (as often as possible), and when I've had a crash, I generally have lost minimal input. The downside is if you totally screw something up (over-zealous deleting, anyone?), you can't go to Detail, Abandon Changes and undo it unless you notice really quickly.
One of my carryover notes for one of my clients has a detailed list of proforma'd things that should be deleted (prior-year IDCs from pass-throughs because I calculate excess IDCs at the entity level, state source passive carryovers for AMT because none of the states involved have a state-level AMT), which things shouldn't be deleted (state-source passive carryovers for regular tax), and how to get the amounts to key back in when you delete the wrong thing.
I find the automatic error checking (those little red !s in the Detail screens) slows my computer way down, but I've never noticed a significant lag due to auto-save.
I use a NAS, and Lacerte's performance when using it (and they do NOT support it) has slowed substantially in the last two years.
I've tried playing with the auto-save, and with the error notification (red dots) as well as not showing appointments on the display. They all seem to help.