All right, so these last days I've been reading a little about relativistic speeds and time dilation online. Most of it makes sense to me, but there's just a little thing that I've been unable to wrap my head around. When you travel at near light speed time slows down, so when you return to Earth you may have been gone for a year but two years have passed on Earth. This I've understood, but now let's imagine a spaceship travelling from Earth to the nearest star 4.37 lightyears away. Travelling at 90 percent the speed of light this journey would take 4.86 years. Due to time slowing down to the people in the spaceship, time moves roughly 2.3 times faster outside the spaceship (used some online calculator for this, don't know if it's true), so for them the journey takes 11.17 years, is this right?

If it is, then what speed is the spaceship really travelling at? For the people on the spaceship the speed is 90 percent the speed of light, but for someone outside how can it travel anywhere near that speed when it takes more than twice the time to reach the destination? I'm sure I've critically misunderstood something somewhere, but can someone please try and tell me what?

Also, try and keep it simple. I'm not stupid but I'm no scientist either, big equations scare me