Is it necessary to let the starter double before adding it to bulk ingredients?

It's not just to check if your starter is strong, you wait until the start is ripe so that the yeast/bacterial population is big enough to get the job done in a reasonable amount of time.

Let's use the following basic recipe as an example:

Levain

• 100 g flour
• 100 g water
• 20 g starter

Main dough

• 200 g levain
• 900 g flour
• 600 g water
• 20 g salt

Hypothetically, let's say that the levain takes 8 hours to become ripe, and bulk fermentation of the main dough takes 4 hours. If you don't wait 8 hours for that levain to become ripe before mixing it into the main dough, you are essentially creating the following recipe:

• 1000 g flour
• 700 g water
• 20 g starter
• 20 g salt

And instead of bulk fermentation taking 4 hours, you're looking at it taking 12 or more hours as the microbes in the starter still have to multiply enough to produce the gas and acids we're looking for. If that timing works better for you, great! I myself prefer a low inoculation (50g stiff starter to 1000g flour) and long bulk fermentation at room temperature over a large preferment with a fast bulk fermentation, but you can't just drastically change the microbial population of your main dough without expecting some changes to the process.

Allowing the starter/levain to ripen before use ensures that it contributes plenty of yeast to the dough.  In other words, it actually helps with your objective of saving time, since a dough made with an underripe starter/levain will rise slower.

If fast bread is important to you, you will probably prefer using bakers yeast instead of sourdough since sourdough is the opposite of fast.

Paul

if you think it's strong enough - use it - adjust as needed. Enjoy