Most golfers obsess over the line and barely think about speed. Backwards priority.
A putt hit with the right speed on the wrong line often still has a chance — it can catch an edge, a lip-in, a favorable bounce off the cup. A putt hit with the right line but the wrong speed almost never goes in. Too hard, it blows past the break entirely. Too soft, it dies before it gets a chance to curve in.
Next practice session, putt with your eyes closed on the last few feet. You'll feel speed in a way your eyes never taught you.