The price you must pay for short strength workouts to be effective is hard work.
This means putting a really high degree of effort into each set you perform. The problem is most people don’t do this. At the first sign of discomfort they quit.
What is a high degree of effort? Ideally you should reach a point where you are unable to complete another repetition with good form. In the fitness industry this is commonly known as reaching momentary-muscular-failure or MMF.
MMF is highly uncomfortable and your mind will be willing you to stop before you get there but the final few reps, tough as they are, hold the greatest potential for triggering a positive adaptation in your muscles.
Consider the rest of the repetitions a warm-up for this moment.
How do you know if you have reached MMF? Always attempt one more positive rep. If you fail to move (despite your best intentions) for at least 5 seconds positive MMF has been reached.
As safety note, you want to make sure you are choosing resistance exercises which allow you to fail safely. The Gymless Training System contains loads of them!