Imagine if you can how fast we'll be able to travel in space another 300 years from now.
For that, we'd need a whole new source of energy. Doable, but unlikely without some major breakthroughs. So far, we've been using primarily stored solar power (mainly in the form of hydrocarbons) to fuel our advancements, with a sprinkling of supernova remnants (nuclear fission). To put it into perspective, in 100-200 years, we're pretty much expending about 1-2 billion years of accumulated solar energy. Forget progress, in order to sustain civilization at this level in 300 years, we'd need an easy form of energy that rivals what we have now. And right now, nothing comes close.
And this is ignoring the societal-environmental backlash that's coming upon us fast. That we might create some kind of technology that can increase our energy production is assuming that society remains stable enough through the next 300 years to allow this. There are a lot of factors at play, and my bet is that we're not going to see the kind of growth that we saw the past 200 years in the next 200 years. We'll be very lucky if we don't regress as a whole (for certain, the wealthy will progress but the bigger question is whether the middle class and the poor will follow or if they will suffer to allow the wealthy to progress).
Quite frankly, this is a hurdle every alien civilization will face. The farther you want to travel, the more energy required. And considering we haven't been invaded by aliens yet, I would imagine this to be a more difficult challenge for everyone than you'd expect.