A great deal of personal productivity focuses on managing time. When and what tasks to do; making sure you're spending your life's currency as you intended. How long something takes, what to do first, what to leave for later. This makes sense as our time available is the ultimate constraint - we can't get it back or make more of it. One thing in common is we all have 24 hours a day until we eventually don't.
The problem with focusing on time allocation is that we miss what makes an hour more productive. Two hours can be used for different things and can yield orders of magnitude different results. We can very diligently spend time doing busy work. We can also be 10x more productive with the right mindset and approach.
Intensity depends on focus and energy. High energy people can get more done. This sounds like a personality trait, but it can be managed and cultivated. Anyone can become high energy if they put their mind to it. Think about things that cause positive emotion - a bit beyond what's 'normal' for your day. That song, joke, excitement about a new release, blog post, video. We all have stuff that raises our mood and gives us an extra boost. This isn't physics: there's plenty of it because it's made up.
Be proactive about boosting your energy. Then manage your time according to your energy level. More will get done with less pain.