Does the Granny setting increase charging time?
The 5 amp setting will increase charging time by a factor of 2.4 vs the 12 amp setting.
Where does the term "granny" come from and what is its significance?
Both my 2017 and now my 2019 Bolt EVs had a choice of 12A or 8A at 120v; I do not know where a 5A setting comes in play?
You can elect to have the default of 12A when at "home" (and this setting will expire in 90 days) but everywhere else you must remember to manually select 12A when you charge at 120v, otherwise it will go out at 8A.
Charging time at 12A/120v is slow and at 8A it is excruciatingly slow; it takes about 50% longer to charge at 8A than at 12A.
The choice of 12A vs. 8A is strictly related to the ability of your host to provide the amperage needed.
For example at one motel which allowed me to use their 120v outlet for charging their golf carts, 12A was easily available. At another motel, their 120v outlet was shared with the landscape watering controller so I had to limit charging to 8A.
There have been posts that suggest a DIY adapter that will allow the GM supplied charger to work at 240v/12A, which of course reduces the charge time substantially, if 240v is available. DO NOT CUT OFF THE ORIGINAL 120v PLUG, as it has various sensors contained in it for your safety. Use a DIY adapter, if 240v is available.
I carry the 120v/240v 16A AV Turbocord charger (which easily packs into its included zippered small footprint carrying bag) https://info.evsolutions.com/turbocord
, heavy duty 120v and 240v extension cords, and an assortment of 240v plug adapters, so I am able to charge at most any 240v outlet I encounter (for example a dryer outlet in a garage or the 240v/50A available at the Flying J Truck Stop at the top of the Grapevine).
I have to admit that with the proliferation of Level 3 DCFC chargers now available to me in California, I rarely use the AV Turbocord anymore.