The definitive answer is "it depends on you, are they?"

I know that isn't helpful but lets unpack a number of things from your question and address them separately.

First lets talk about skill. You said "I read slowly and I don't remember everything I read either."

In my experience most people don't remember everything they read, but they do remember the "big points" that a book was trying to make. I say this to reassure you to not get hung up on how much you retain, you can always re-read the book and get more out of it. But the other part was you said you "Read slowly."

Reading is a skill, like bicycle riding or playing the piano, or shooting baskets in basketball. The more you practice it the better it gets. A strategy I used with my kids and with folks who are new to reading is to find books that are not too challenging to read, and are engaging which means you want to read them. Science fiction stories, mysteries, any of a number of categories of fiction meet the bill. Go to your nearest library and ask a librarian for some suggestions on things to read that have good stories. Ask for skill ranges from early reader to college level. Check them out, read them all, and make notes of the ones that were interesting, the ones that weren't, the easy ones to read and the hard ones to read. No need to write a book report just write down "Read '1001 Arabian Nights', found it entertaining but kind of boring." or what ever your take was on the book. Since you aren't "learning" from these books, just practicing your reading skill, you should not be anxious if don't remember all of the details or even who the main characters were. Go back to the library and give your take on the books to them and ask them to help with picking out 5 or 6 more. Rinse and repeat.

What this does is it hones your sense of what you find "fun" to read. And when it's fun, just like video games, there isn't really a question of where does the time come from. But let's talk about time for a minute.

You called out that books were a "big time investment." Which is true, it is one way to invest your time. What are your other time investments? Have you done the experiment where you journal how you spend time every day for a week? Doing that will give you insight into what you are spending your time on now and give you an idea of what you are "investing" in. Reading will develop your vocabulary, sense of humor, conversation skills, knowledge, and mental acuity. That is what you get by investing in reading books, compare that to what you are getting out of your current investments and perhaps that will inform your choice on how much time you might spend. Also it is useful to remember that you can start and stop reading easily, carrying a book on your commute (if you aren't driving) is an easy way to get extra value out of your time, you are both going or coming from work and you are improving your reading skills! bonus!

Then there is this: "I always assumed that everything could be found for free online." This isn't ever going to be true. Not the 'free' or 'online' part, the found part. While most information ends up online, either legally, quasi-legally, or illegally, finding it is becoming exponentially harder as various forces work to monetize, hide, or discredit it. Having worked at a search engine that indexed billions of nominally "good" web sites I can tell you that our experience was that 99.999% of it was crap. Finding the good stuff is really really hard and getting harder. And it moves so finding it a second time can be just as hard.

The bulk of the things you want to know can be found already written, in books, by someone who spent time collecting the information, verifying its veracity, writing it up, having an editor help them rewrite it for clarity and having a reference department check it for accuracy. You can check those things out at a library for free. You can buy them for small change at rummage sales and yard sales, and you can share them with others by swapping books.

What I've found as I've gotten older is that books for me fall into three categories, entertainment, working knowledge, and reference knowledge. I found the "Dune" series by Frank Herbert very entertaining and keep a copy on my book shelf. I have a copy of "The Art of Electronics" with post-it "flags" at sections I often refer to as part of my working knowledge set. And I've digitized (scanned into OCR'd PDF files) dozens of text books which I think of as my "reference" set. I use a simple search engine to look up specific things in those books.

Bottom line: Work with a librarian at your local library to beef up your reading skills and ability to use a library. Then use those skills to read books that make you feel happy or more useful or more knowledgeable. You may find, like I did, that reading is more fun that watching television or playing video games. Or you may not. Again, it's a question only you can answer.