TKD sparring competitions are based around speed. A TKD practitioner who brings TKD sparring to a fight is the guy who gets KTFO'd. This is the majority of TKD practitioners as most train for the enjoyment of sparring competition and feel that it directly applies to fighting.
IronMan is exactly correct in his statements that most TKD fighters striking isn't a threat and just an opening for takedowns. Most TKD fighters never train takedown defense.
The whole premise of TKD striking is that your throw your attacks with so much force that your opponent has to either block or avoid them entirely. Most TKD practitioners don't understand this concept at all.
IronMan is also exactly correct that you have to establish the credibility of your striking quickly in a fight. Dancing and pawing works if you are looking to win on points, but typically not if you are trying to finish a fight. These are the fighters that are winning right up until they get KTFO'd.
Most TKD fighters in my experience get real hesitant in a fight where they know they can be clinched or taken down. A hesitant striker is one just surviving until their KTFO arrives.
TKD striking can be every bit as effective as Sanshou or Muay Thai provided the appropriate translation takes place between sparring and fighting and the TKD practitioner trains for fighting. High level TKD fully employs eight point+ striking like every other credible striking art.
TKD speed training or 'flicking' kicks and punches is a total failure in fighting. Punching power is magnified by your triceps, shoulders, back, hips, legs, ankles, even your toes. End discussion. Scientific fact. Stated previously by IronMan and linked from credible sources.
Strikes are measured in terms of force, typically PSI (Pounds per Square Inch)
Acceleration = Final Velocity - (Initial Velocity/Time)
Force = Mass X Acceleration.
Power = Force X Displacement
TKD striking based on speed means that TKD has the potential to generate large amounts of force. What most practitioners do not understand is how to turn that force into power. Transferring the energy of a strike into your opponent is what powerful striking is all about. This is accomplished via kinetic linking.
Striking is about energy transferrence!
A trained, aggressive, confident, experienced, and powerful TKD practitioner should be very respected in the striking game.
I'm 6'6" and weight 235lbs. If I need to train power it is difficult for me to find someone to hold heavy pads for my kicks. Through heavy pads I can still cause significant bruising. I could never use that training for TKD sparring competitions but the fact remains that I'm still using TKD fighting technique and not TKD sparring technique. I've caused verbal submission multiple times from leg kicks and even blocked higher kicks. Rarely are my kicks caught because people don't want to risk a broken arm or ribs. Every kick I throw I throw with 100% commitment and with full awareness that I could taken down. If my kick is caught I'm ready to jump into guard or attempt a throw/submission. I would rather throw with intent and pay the costs than go through the motions of any strike.
TKD is not a hesitant art, period. The art provides the necessary tools for MMA and fighting in general. Most TKD practitioners get destroyed because their first mistake takes them almost entirely out of their game. I've seen BJJ practitions who aren't familiar with MMA grappling get destroyed because they drop for a knee bar and get their head pounded in because they aren't familiar with punching in competition. I've seen multiple wrestlers taken out of their element when they take a knee to the face or end up in a guillotine. Every art still needs to be adapted to MMA but the pure striking arts typically have less of a margin for error and are highly criticized because mistakes are typically more costly and more obvious.
This was a bit of a ramble as I'm working and didn't take the time to format my post.