> There's an entire class of hobbyist electrical engineering - retrocomputer electronics - where an autorouter is essential (old computers used wide but slow busses, so routing 50-100 traces across a pcb is what matters, signal integrity does not).
You can get away with murder at 1MHz if your trace length isn't too bad but you absolutely do need to worry about signal integrity over that speed or large boards.
When I was a beginner starting out with 65C02's my autorouted boards would work fine around 1MHz but it was impossible to get them working at all at 10MHz until I rerouted the board by hand taking care to think about signal integrity, current return path etc.
In my experience, you can get away with a lot up to 10-25MHz. Mostly just proper board stackup (the bare minimum ensurance that all signal traces are on a layer that's next to a solid ground plane layer, with no slots or cutouts under the traces).
Things like trace length, reflections, power dips when the bus is all 1's, electro and/or magnetic crosstalk, etc. become an issue at around 50MHz, although I've never gone that high in a hobbyist design...
You can get away with murder at 1MHz if your trace length isn't too bad but you absolutely do need to worry about signal integrity over that speed or large boards.
When I was a beginner starting out with 65C02's my autorouted boards would work fine around 1MHz but it was impossible to get them working at all at 10MHz until I rerouted the board by hand taking care to think about signal integrity, current return path etc.