Tie their shoe laces together?
In all seriousness, your best bet is to use their speed against them. It doesn’t matter how fast they’re running, if they fall over they won’t be running for much longer.
By far the simplest way of stopping them is to dig a 1ft by 1ft by 1ft hole, essentially a rabbit hole. If you’ve ever stepped in one, you’d know how easy it would be to twist or break your ankle. Now, imagine you’re running at these high speeds, not a care in the world, and step into one of these holes. At best, you’ve twisted your ankle and torn the tissues in your leg, you might instead break your ankle, your leg, damage the nerves, tendons, muscles, perhaps even the artery in your leg. A potentially fatal but easy to construct trap.
Going slightly bigger and more advanced, you could have the good ol’ pit fall trap, a large pit covered by a tarp and hidden using sticks and dirt. The speedster runs over that, they’re going too fast to stop and falls in the hole. The fall may not be lethal, they may just break their legs in a cruel twist of fate, but you can easily make it lethal by putting wooden spikes at the bottom.
Further options are trip wires, caltrops, raising low-barriers or wet/slippery surfaces. All of these would cause major issues for a speedster. I also remember a story, i don't know its true though, where during the second world war, piano wire was tied to trees or lampposts so that the wire stretched across roads. This wire was tied so that, if you were sitting down, it would be at neck height. The idea of it was to kill Nazi motorcyclists by decapitating them as they drove past, using the speed of their bikes against them. Because of how thin piano wire is, you wouldn't be able to see it and, by the time you did, it would be too late to stop. As i say, i dont know if this was true, its just something i’ve heard.
Taking inspiration from that idea though, you could have walls of clear glass, like French sliding doors. If a speedster ran through it at high speeds, they might be cut to ribbons by the shards of glass and have glass in their eyes.
If you were talking about in combat though, i suggest pike formations. Speedsters, like horses, aren't stupid enough to run into this big, sharp pointy stick. However, if you had these hidden in long grass, for example, and lifted them up when they got close, they may not be able to turn back in time and be skewered.