In an interview John Wesley Shipp stated that the studio hired over 100 mimes to help with the "Flashtime" scenes where the extra were to remain frozen in time but in those scenes CGI wasn't an option due to all of the interaction between moving and frozen people in the episode.
The concept of Flashtime was created for the series.
Barry calls Iris his "lightning rod." This is a reference to the concept introduced in the comic limited series "The Flash: Rebirth". A speedster's "lightning rod" is his or her loved one who can help him or her escape the Speed Force and return to a certain point in time.
In DC comics, Veronica Dale is an eco-terrorist known as Hyrax an enemy of Green Arrow. She is the leader of Eden Corps, though the group is instead focused on animal rights activism. In her brief appearance, a bomb that she detonated killed Oliver Queen.
The equation written on the board in the speed lab at the beginning of the episode--3x2(9yz)4A--is the speed equation that Barry created to increase his speed in order to defeat Zoom, the equation he went back in time to get help with in episode 2x17 "Flash Back." (Other similarities between that episode and this is the inclusion of "Flash" in the title, the use of a new time-based power (time travel in 2x17, flashtime in this), breaches being all over the city created by the villain of the season, even Harry being the main Wells of the season.)