Warmups - are they a time killer?

No they are not. They are an extremely important precursor for your workout. Warm ups lead to an increase in muscle temperature and blood flow that in turn leads to improved performance and reduced risk of injury to muscles and tendons. Stretching is a part of warm ups that enhances the range of motion of the joints ensuring flexibility. 

A warm up session usually lasts from 5 to 15 minutes before beginning the main exercises. It reduces the loads on the heart which otherwise can be a sudden surprise resulting from high-intensity exercises. It reduces muscle stiffness and increases elasticity. Warm ups function on a psychological level as well by imparting preparation, stability and the confidence for performance. It instructs the brain to stay put for intense physical activity. Going by the definition, the term warm up is explained as a period of preparatory exercise to enhance subsequent competition or training performance. For sportspeople and athletes, it stimulates the blood flow to the periphery and increases muscle temperature for more free and coordinated movements. An important factor to note here is that the warm up alone should not result in fatigue. It should be kept in mind while formulating the set of movements. They should be able to cater to all the body segments. 

There are specific functions and objectives that a warm up caters to. 

1. Alert different organs in the body to function
2. Grant different body muscles with the required flexibility, ease and efficient mobility
3. Increase heart pulses rate and amount of blood required for every pulse
4. Regulate and speed up respiration, as well as to speed up blood circulation
5. Raising body muscle temperature
6. Reaching the maximum capacities of the body to respond to any actions demanded
7. Positive emotional reaction for training practices and competitions
8. Maximum psychology alert to participate in training or competition.

The activity of warm up generally consists of a gradual increase in intensity in physical activity,  joint mobility exercise, and stretching, followed by the activity. Warming up leads the body into a condition at which it safely responds to nerve signals for quick and efficient action. For example, before running or playing an intensive sport, the athlete might slowly jog to warm their muscles and increase their heart rate. It is important that warm ups be specific to the activity, so that the muscles to be used are activated. While exercising, we perform a tough effort, our muscles ask for additional oxygen to be able to do the effort, additional oxygen comes through regulating and increasing speed of breathing and speed of blood circulation. Simultaneously, metabolism has to be increased to keep up with other body parts speed. Therefore, individuals should increase their efforts gradually so that the body's internal organs function can keep pace with the player's effort and movement. We must prepare our body's internal organs adequately to fit with the expected physical effort and to prevent any threats of common injuries.

REFERENCES:

Park, H. K., Jung, M. K., Park, E., Lee, C. Y., Jee, Y. S., Eun, D., Cha, J. Y., & Yoo, J. (2018). The effect of warm-ups with stretching on the isokinetic moments of collegiate men. Journal of exercise rehabilitation, 14(1), 78–82. https://doi.org/10.12965/jer.1835210.605

Fradkin, Andrea J1; Zazryn, Tsharni R2; Smoliga, James M3 Effects of Warming-up on Physical Performance: A Systematic Review With Meta-analysis, Journal of Strength and Conditioning Research: January 2010 - Volume 24 - Issue 1 - p 140-148

doi: 10.1519/JSC.0b013e3181c643a0

http://www.ijsrp.org/research-paper-0116/ijsrp-p4923.pdf