[рони]

Ops,
как вариант ...

let timer;
        $$('.up, .down', box).forEach((el) => {
            el.onmousedown = () => {
                window.clearInterval(timer);
                let tt = this;
                timer = window.setInterval(() => {
                    tt.activatenumber(el);
                    tt.sumForm(form);
                    form.dispatchEvent(new Event("change"));
                }, 350)

            };
            el.onmouseup = () => {
                window.clearInterval(timer);
            }
        });