What kind of mission? If it's a pirate that needs killing, try dropping in the nav beacon or cruising slowly, they tend to come to you. If you have a FSD interdictor, you can also often fish them from SC - just be quick, they try to run.
... I have also found that jumping to a nearby system and then back again sometimes makes the mission target spawn nearer.
That's massacre mission... you don't know where to go until you entered the system.That's what the Abandon Mission button is for.
For assassin mission, yep 99% of time they come to you the moment you enter the system, no hussle, easy money

No wonder why this mission pay me 12mil for just 20 pirates..... well, I head to the HazRes 1k ls near by instead.