According to the link, the local service trains will see the biggest improvements from electrification (100 minutes -> 75 minutes). This makes sense because my understanding is the the electrified system is quite a bit faster in accelerating which really helps when station stops are only a couple miles apart [0].

The difference on the express service is much less (65+ mins to "under an hour"), presumably since the trains can spend more time at or close to top speed due to longer distances between stops.

[0] You can see a list of stations and mile markers here: https://en.wikipedia.org/wiki/List_of_Caltrain_stations