"extensive background investigation", and they are serious. My Q clearance is long gone, but even in the 70s/80s the FBI interviewed ALL my past faculty, ALL my hometown neighbors with "X" blocks of my home, asked everyone about drug use, womanizing, poked into all my finances, background checks on my family, and I’m sure more than I'll ever know. They wanted to know anything that someone may be able to hold over my head as blackmail, etc. In those days an “L” clearance was around 1-3 months and a “Q” could take 6 months.
At the ripe old age of 57, my brother hit the FBI’s mandatory retirement age and was thinking about things like this. According to my now permanently retired brother, the “newer” security evaluations includes all your immediate family finances, including bank balances, investments, etc. (parents, siblings, children, and the same on your spouse’s side), with the aim to I.D. folks getting paid for leaks via family. Generally, I suspect the clearance will come with the “you give up your right to x, y, z legal protections”. I remember the old “Q” had some language to that effect.
In short, this is no small step. My brother was never bored, but it was nothing like the FBI on TV. I suspect the CIA would be similar for someone that likes investigative work/research. However, based on my brother’s experience, your attention to detail and documentation of everything in government intelligence better be at the highest level. But for the mandatory retirement age, he’d still be there. Politics are serious in play, however, so thick skin is a must. You will never be able to discuss anything about your work, so you will never impress friends, your children’s friends, etc.