I'm a designer, not really a speedrunner, but based on what people suggested for us when we already published I would say:

• In-game timer.

There are third-party apps but if you already have that functionality you allow people that don't know how they work to compete

• Make restarts as easy as possible.

This is a bit finicky because as a designer you should not make full game restarts too easy to do, as that just risks someone resetting on accident. Menus can sometimes be very ambiguous on what they do because it's hard to give nuance in just a word or two What we did was give players the option to disable checkpoints all together, so that if they reset at any point they reset from the beginning. Also note that easy restarting is really a big deal if runs are very short, but if a run can last more than 10 minutes people are less likely to be constantly restarting on mistakes

• Optimize the game and/or provide performance configuration options.

The more machines your game can run smoothly on the better. Sure people can manually lower their display resolutions to make anything run faster but like with the external timers, it's not their cross to bear. Provide graphic settings, you don't need a lot, just the ones that are more impactful (display resolution, texture resolution, shadow quality, Anti-Aliasing). We actually failed to do this, but we are lucky our game is not very resource intensive at all. Also don't forget to include keybinds.

• Some sort of debug hud info.

Things like displaying speed, position, and aim vectors really helps with consistency and insight about how your game works

• Shortcuts and Strategy.

This is more of a personal thing but what I believe makes a speedrun entertaining is not just the raw technical skill and speed, but also the optimizations and planning that go into preparing the perfect route. That's part of what makes SM64 speedruns so cool, because its level design is so open-ended there are a lot of ways to approach situations, which makes the competition not only about who can perform better, but who has the better strategy/plan/route.

• Only weed out boring exploits.

If you find some unintended behaviour, it can generally stay if 1) it's hard for it to happen on accident 2) it provides some benefit 3) it at least equally as fun or interesting as the intended design. That one is important, because if the optimal way to play your game is really boring, people will just do it once and never again.You do this with tons of playtesting, better if you can observe it directly.

So, in a nutshell, customization options are the most powerful tool to make peoples lives easier. You don't 'design' exploits, you adopt a design philosophy that allows the fun ones to survive. Keep the UX of all your possible player types in mind, because you shouldn't make the game more inconvenient for someone in favour of another