I'm a bit unsure if there's other factors to the enemy spawn rate. Is it the enemies on the whole map? Is there a cap to how many enemies are on screen? etc.

I am not sure if there's a limit for enemies on the screen, but I'm fairly sure there's a cap for how many enemies can be on the map at once, from trying to do the ricochet achievement, but it's pretty damn high and unlikely to be met in a speedrun.

I've wondered if reaching the cap then using dragon slayer with the tactical visor would be quick, but in my limited testing I found that enemies appear to trickle in slowly afterwards, making it worse than traditional methods.

I've tested a bunch, and it seems like the faster you kill them, the faster they come. I think the game aims for a certain amount of enemies in the arena, and spawns more if the number isn't reached. For example, if you don't kill the first four, nothing will spawn. Only when you have a kill count of 3 will the game have 5 enemies in the arena. At 4 kills, it'll aim for 6 enemies, and so on. Which means killing them as quickly as possible after spawn is the way to go in default%, with pistol if far away. In any% it might be more efficient to save dragon slayer for more enemies.