First make the rules properly well defined, and introduce some consistency in the enforcement of them. I mean i try hard to believe that F1 is not rigged, but that's hard sometimes.
Second, stop all the ludicrous cost-cutting business which has done all but the name, and stop making F1 a standard and endurance championship. 1 engine and gearbox per race weekend. And stop that bullshit tyre-rule. Too many races have been decided in qualifying and 1st stint. The 3rd stint is almost always just a procession.
To control the speed use stringent rule on aerodynamics and have a joule-limit pr. race. This is an easy and non-intrusive way to manipulate speed.