Tetromino Per Seconds, Tetromino Per Minutes, Pieces/Sec, Pieces/Min, BPM (Beats Per Minute / Blocks Per Minute), etc are units of speed often used by players to discuss the rate at which a player is processing incoming pieces. Generally, games and players in English-speaking regions, and TTC prefer minutes as the base time unit, while those of Japan prefer seconds. 
As most games do not have a built-in speed value display, the value is often manually calculated from replays or video recordings of gameplay. Some games, such as Lockjaw, or Quadra have speed value displays within the game itself. However, the algorithm to calculate the values varies between games. Most replay/video calculation methods and Lockjaw take into account the time spent during in-game animation such as line clears, while Quadra above version 1.1.3 do not. This causes Quadra to report higher values than that of other games, making speed values between Quadra and other games not directly comparable.
While speeds of 3 tetrominoes per second or 180 tetrominoes per minute and upward are generally considered fast, it is important to note that duration is also a factor when looking at tetromino per time values. It is possible for anyone to artificially inflate a tetromino per time value by only measuring it across a very short span of time, such as several seconds. Generally, the longer the timespan for the value, the higher the probability that the player is able to sustain that speed.
Although less popular, some games such as Tetris Zone, Tetris Online (Japan), Quinn and Tetrinet clients include measurements of lines cleared per minute. Multiplying this by 2.5 gives an approximation of tetrominoes per minute, as there are 10 blocks in a line and 4 blocks in a tetromino. This is slightly less accurate, especially over the short term, since it does not take into account the amount of blocks left behind in the well.