After The Flash and the Reverse Flash fought each other in the year 2000, the Reverse Flash was seen losing his speed. Gideon (who surprisingly has not been seen at all so far in Season 2) informed the Reverse Flash that the jump in time drained his powers.
But maybe there was more to that. Perhaps prior to the great fight, the Flash and the Reverse Flash felt the need to set aside their differences and team up to work on a major problem: Zoom.
It is possible that Hunter Zolomon (Zoom), who can control how time flows around him, wanted to become a speedster. In order to do this, he decided to take up the mantle of the Black Flash. This would explain Zoom’s demonic characteristics.
In episode 8 of the Flash, Dr. Wells suggested that Zoom is four or five times faster than the Flash. This could be a reference to the Black Flash in the comics, because the Black Flash is rumored to be four times faster than the speed of light, while the Flash is usually considered to be as fast as the speed of light. (Granted, the Flash in the show cannot run at the speed of light, but that would be four times faster.) In addition, Jay Garrick has referred to Zoom as a "demon," a term that is traditionally used to refer to the Black Flash. One of the producers of the show, andrew kreisberg, described Zoom as coming from "hell." This could be a hint at Zoom taking up the mantle of the Black Flash.
After becoming the Black Flash, Zoom became insane and wanted to absorb the speed of all speedsters in all timelines and kill them. He feeds off death and the more speed he absorbs through his fingers, the faster he becomes. This was such a problem to the Flash and the Reverse Flash, that they were forced to work together to take down Zoom.
While the Barry and Eobard were working together on Zoom, Barry revealed his identity to Eobard. But during an intense fight with Zoom that included Barry Allen, Jay Garrick, and Eobard Thawne, Thane got pinned down by Zoom. Barry Allen hesitated to rescue his old enemy. Jay Garrick, however, went over and appeared to knock out Zoom, saving Eobard.
They then thought that they had defeated Zoom and Jay Garrick left. Eobard was furious with Barry for allowing Zoom to steal some of his speed. This led to a massive fight. During their fight, the Reverse Flash traveled back in time to kill child Barry in the year 2000. The Flash knew of his plan and used technology from Rip Hunter to follow the Reverse Flash back in time to his childhood. This allowed him to exist in two places at once.
Due to the speed that was sucked out of him, Eobard’s grip of the speedforce was weakened and collapsed after time travel and a nasty fight with the Flash.
As a result of their time travel, the two speedsters disappeared from that timeline. This prompted Zoom to travel back in time as well. He started in Earth 2 and took out Jay Garrick. Earth 2’s particle accelerator was likely meant to explode at that point in time, whereas Eobard altered the timeline for the Earth 1 accelerator to explode. Now that Zoom has found Barry, he wants to finish him.
Using the resources that the Black Flash has, he knows Barry’s identity and everyone that he works with. He can see through time, and therefore, could easily take down Barry Allen. However, Zoom is still human enough to want the pleasure of dragging out his fight with Barry. This is why Zoom doesn’t just travel back in time and kill him. In addition, the faster the Flash is, the tastier his speed is. This also why Zoom sends over metahumans to fight the Flash. He wants his prey to be as tasty as possible.
Taking down Zoom alone will prove to be very difficult for Barry. In order to beat Zoom, he will likely have to team up with multiple speedsters.