There could also be a small time jump for next season. Really the best explanation is probably the show just doesn't care much about distance.
I would say they used to care, but don't anymore. Think how long it took Ned, Arya and Sansa to get from Winterfell to KL in season 1. Or how long Arya traveled with Yoren. They have sped up the story in the last couple seasons and have opted to just ignore the travel times and time jumps and let the viewer figure it out for himself/herself.
With that said, to the viewer, it didn't seem like much time passes between when Tyrion said goodbye to Varys (as he left for Westeros) and when Dani reappeared on top of the pyramid. The city was already under siege, and she unleashed the Dragons the next day (I think). Then, they had their ships and started preparing for transport across the narrow sea. Next thing we see, they are sailing and there's Varys who had time to make some deals/schemes, and make it back to Mereen in time to set sail again. It seemed strange at the time.
But....
Let's remember that Varys did not know that Dani was going to get ships from the slavers and from Theon/Yara. So, it seems realistic that he brought transportation (Dornish and Tyrell) with him to bring Dani's army back to Westeros. Now, he would not have brought enough ships to transport the Dothraki horde, but, fortunately Dani had acquired other ships to transport them.
Additionally, did you notice that most of the ships had prows carved to look like Dragons? That work would have taken months to finish. So, while it doesn't feel right to the viewer, HBO is leaving clues to let us know that these events are taking place over many months and we are only seeing the highlights.
