Selector

You can download a similar Illustrator file here and try the desired selection:

https://drive.google.com/file/d/1DU1n1utWQb9BxA5LlMEs46udtU0Dqh_T/view?usp=sharing

The question remains:

About how long does it take for you to reliably select all objects that are completely located inside of paths P1, P2 and P3 (see "Selector" layer in the Layer panel)?

Are the line segments that are outside of the green lines separate from the ones that are inside of the green lines? If they are separate then you could select them and the green lines first and then lock them (Command/Control-2). Then a simple "Select All" or marquising with the regular selection tool would select all of the rest instantly. Since the posed question only referred to the lines inside of the green lines how long it would take to select the lines that are to be locked would be irrelevant.