I'm fortunate enough to have been GMing long enough to have become a fast improviser. The only time I couldn't improv a scenario faster than my players could experience it was when we we're in the Elemental Plane of Fire and all the elementals had clever names, which i couldn't make up on the fly. Supposedly, it was the only time they ever felt they had the upper-hand, and asked every passerby their name.
That being said, I run open world stuff mostly, so its a lot of exploration on the player's part. Generally I work Large Details to Small. I will extrapolate the world based on what i know about the greater world around it; Asking questions like, What kind of City would be in this kind of region, what kind of notable locations would be in this kind of city, what kind of notable people would be in this kind of location, what are some significant events that would have made this person who they are? Players will gravitate to their interests and you don't have to have every answer, you just need to know the formula for getting the answer.
When I was younger i would spend days making maps, then recycle those maps for future campaigns. These days I go completely Theater of the Mind, and use playing cards as a map. Players don't mind, especially if you can build a vivid scenario. Fill the room with more details than needed, include senses other than just sight, and allow players to fill in the gaps with their own imagination.
The most work I do is within the few hours before the game starts. All I do is write down single sentences that remind myself of the ideas I've had over the week. What kind of information I want to give the players, what events happen, which character to introduce, etc.
tldr
Know your world, sign up for improv classes. Hope this helps.