Speed is a character type that groups together characters who can run fast, or those that excel in their ability to be able to play a sport (such as tennis) quickly.
Games in which Daisy is classified as this Character Type Edit
In Mario Kart DS, Daisy's Standard Kart DS had 6.5 units of speed compared to Mario's 6 units of speed.
Outside of potential short scenes in games such as Mario Party, the first game to explicitly label Daisy as a Speed type character would be in Mario & Sonic at the Olympic Games where she and Yoshi were the only two characters on Team Mario to be classified as Speed types. In the first Mario & Sonic game, Daisy is actually the slowest of the Speed characters and is tied with Peach (who is a Skill character) for speed, but regardless Daisy is one of the fastest characters. Later Mario & Sonic games would continue to classify Daisy as a Speed character. Mario & Sonic at the Rio 2016 Olympics Games doesn't divide up the characters into character groups, but Daisy is still a Speed character.
Mario Strikers Charged would officially label Daisy as a "Defensive" type. This game had types ranging from Offensive, Defensive, Power, Balanced, and Playmaker. Defensive and Playmakers players are the fastest characters of the bunch. Thus, Daisy - while technically called Defensive in this game - shows her Speed on the field in Mario Strikers Charged. Daisy is one of the fastest captain characters, along with Peach, Waluigi, and Diddy Kong.
The next game to classify Daisy as a speed type character is Mario Kart Wii, where characters were divided into three size classes: Small, Medium, and Large. Daisy is in the Medium class, While Medium characters are more balanced than the other size classes, each character has their own bonus stats. Daisy received the best speed boost of the medium characters, leading to her being used in Time Trials and online play by many competitive players. Along with Funky Kong, many world records in Time Trials were done by using Daisy. Baby Daisy also received a speed boost, and is considered one of the better small characters in Mario Kart Wii. Later Mario Kart games don't have hidden stats, so Daisy is not considered a Speed character in Mario Kart 7 or Mario Kart 8.
Outside of the games above, Daisy has not been shown off to be a Speed type character.
☀ While Mario Tennis: Ultra Smash lists Daisy as an All-Around character type, Daisy appears to move slightly faster than Mario, who is also All-Around, which may reference Daisy's Speed attribute.