Walking from 2B to 2A would involve going landside and entering France. Walking actually takes no more than 10mn. The two additional passport checks can unfortunately take some time.

Entering France in 2B can take from 2mn to 30mn, but you can see the queue when arriving in the terminal; in addition, as your luggage is checked through, you can have the choice of either of the two exits into France (Baggage 1 & baggage 2), choice that your fellow passengers coming from PRG will not have unless they have a connecting flight, and this should help you shorten the time needed to enter France.

I have no recent personal experience of outgoing passport checks in 2A. Last year, it did take me 15mn, which seems average, but I was not going to the US in 2A.

The second option is the airside shuttle. Unfortunately, as the shuttles rotate clockwise arounbd CDG2, you will have a stop at 2F and 2C before reaching 2A, so it could take some time.

In any case, you will need to clear security in 2A, whether you have walked or taken the bus. I have no personal experience of the additional checks required for flights to the US.

Your choice depends somewhat on your arrival time in 2B and on the length of the two immigration queues in 2B. If either one of them looks like it will take less than 10mn and your incoming flight is on time, I would personally choose to walk, but others may have a different experience.

Enjoy your flights