A timer can give sketching a weirdly intense feeling. You set a 2 minute or 5 minute timer, you look at your object, and your hand wants to frantically scribble on paper. A timer doesn’t tell you to work fast. A timer tells you to stop solving the whole task.
A timer isn’t a rush signal. You are going to draw a 3 minute sketch of a mug. It isn’t saying that you should have a perfect circular rim, and a shaded handle, and lines showing every imperfection on the surface, you need the height, width, top ellipse, side curve, base, and maybe one shadow shape. A short sketch can give you information if it teaches you what to omit.
Take a few calm seconds to observe before the timer starts, and think about the big, obvious shapes. Is the object vertical? Tilted? Tall, short? Where on the page will you start? For a planter, you might think about a tapered cylinder, and a dark shadow underneath the rim. For a book, a flat rectangular block with one visible side plane. These few thoughts might lead to calmer lines while sketching.
Start light when the sketch begins. Make some loose construction lines to establish the overall shape, make an observation or two, and then add some more construction lines. Lines often get rushed because of dark lines, which are usually placed when there isn’t time to be deliberate enough for the right placement, yet, so you place an outline, a shadow, or an edge and now every other line has to deal with it. Light lines can be moved more easily than dark lines, and they will be easier to cover or refine at the end.
Try doing the same timed sketch three times, with a different focus each time. In the shortest round, capture only the simple form and page placement. In the next round, add the main contour and one proportion check. In the final round, include a small amount of line weight or hatching where it clarifies the object. Doing the same object repeatedly will help take away some of the “I must solve this now” stress.
Don’t feel guilty if your sketch doesn’t feel finished. Sketches that aren’t technically finished are still a huge win as long as they have purpose. If I can draw a simple cylinder, make corrections, and draw one useful shadow, it will be a more beneficial sketch than a drawing full of lots of tiny lines added in a panic.
Once the timer is done, don’t pick up your pencils and start fixing the sketch for another ten minutes. Instead, look at the sketch and see if there is one useful observation to make. Maybe the ellipse was to circular, the shape was drawn too close to the edge of the page, or all of the lines had the same value. This can be written down as an “action plan” on the top of the sketch, so when you start a similar sketch in the future you know what to change. If the timer made you consciously notice something you usually don’t, then it did it’s job.