This is a pretty sticky issue that probably deserves a long answer... and, well, the best I can do here is a relatively short answer.

First, there's some good advice in the RC section of the GMAT Club Ultimate Verbal Study Plan:
https://gmatclub.com/forum/ultimate-ver ... 33279.html. Some of it should be relevant to your timing struggles, too.
Beyond that, you can think of RC timing problems as coming from two sources: 1) you're fundamentally a slow reader, or 2) you're not optimizing your approach to questions. Or a combination of both is possible, too. To be honest, your reading speed is probably pretty "baked in" by the time you reach adulthood, so there might not be a ton that can you do about issue #1 in the short run -- though if you read enough difficult material over the long haul, you might become a faster reader, as well as becoming a better one. But if #2 is the problem, it can definitely be improved.
The most common error I see is that some GMAT test-takers will get stuck in the details of RC passages. And that's really not what you want to do: the details are always going to be there on the screen, and you can always look back at them. When you first read the passage, your goal should always be to understand the big picture: how is the passage structured, and what is the author trying to accomplish? Notice that most GMAT questions are going to be contextual: main idea, or primary purpose, or something about what the author's view might be on a particular issue. They rarely ask you to just regurgitate details, right?
As a baseline piece of advice -- which isn't necessarily right for everybody, but I'll say more about that in a moment -- I'd recommend stopping at the end of each paragraph to ask yourself: WHY is the author writing this? What's the point, what's the big picture, what's the goal? The author is introducing some sort of controversy in a field? Taking a particular stance on an argument? If you don't understand every detail, that's fine, as long as you understand the paragraph's purpose. For most people, writing a few words per paragraph -- just enough to keep you engaged in the big picture -- is enough.
And then keep doing the same thing with the rest of the paragraphs. Ask yourself why the paragraph is there, and how it connects with the previous paragraph. Write a few words, just to keep yourself on track. And if you find yourself writing reams of notes about the details of the passage, you're probably missing the point: you can always look back at those later.
Here's the big caveat: everybody is different. Some people simply read better when their hands are moving, and those people need to take TONS of notes to stay engaged. Different strokes for different folks. But the point of note-taking is to help you be an engaged, active, conscious reader. If the current way you're taking notes isn't helping you do that, then you'll need to try something different.
And keep practicing! That's often the best way to become more efficient.