I think "technique" is a broad term. I break it down into roughly two flavors, probably equally broad and missing some parts :-)

1. Body position: the stuff associated with creating a streamlined, hydrodynamic shape in the water, everything contained in the small tube of your body, etc.
   
2. Propulsion: the stuff associated with generally grabbing more water.
   

Body Position:

• Very low fitness cost to make these improvements. That is, body position is improved by making essentially "free" changes to head position, body alignment, a more streamlined kick (legs not flailing outside of the tube of your body, etc), and more.
  
• This is the "learning how to play a musical instrument" side of swimming: small, and large, technique breakthroughs achieved with focused drill work or 1:1 coaching from a technique coach can achieve huge gains on race day.
• In my experience, and I'm admittedly swagging these numbers, most swimmers swimming slower than about a 1:15-20 IM swim still have gains to be made from improving the technique of body position. That is, if you're swimming a 1:35, for example, you have ~15-20' to be gained by simply turning your barge into a speedboat hull. Doing so should be approached much like learning to play a musical instrument until about 12-14wks out from your goal race, at which you shift your focus over to fitness swimming so you can sustain your new technique for the distance of your race.
  

Propulsive swimming:

• As your body position improves (barge --> speedboat) and you become a faster swimmer, becoming faster still becomes more about better applying your fitness to the water...generally, grabbing more water. This is the technique of the pull, catch, hand position, etc.
• Absolute swim time gains on race day become smaller and smaller, requiring a greater and greater time/fitness/hard work investment.
• That said, in my experience, swimmers between about 1:08-12 can usually get a quick 4-6' swim split pop from finding one small technique tip that just clicks for them. But below about 1:05-6...it's definitely about putting in the work. The sub 1:00 learned-to-swim-as-an-adult triathlete is very, very rare and usually requires a massive time and effort investment.
  

As has been noted, many people in this thread do the math on the time invested vs the gain on race day. Many people make their own time value assessment and decide where they should spend their time, given the other priorities in their lives. For example, a 40hr work week father of 3 does the math on the time investment required to go from 1:15 to 1:10 and makes a decision. Someone else with different considerations makes a different decision.

My general guidance is:

• If you are slower than about a 1:15-20 IM swim, swimming faster for you is generally more about body position and less about fitness, ie, the power needed to grab more water. You should be doing a lot of body position drills until about 12-14wks out from your race, then transition to more fitness-swimming.
  
• If you're faster than about 1:15-20, swimming faster is becoming more about grabbing more water and the fitness/power associated with that.
  
• Sub ~1:10...future gains will be smaller and harder to achieve.
• In my opinion, anyone with limited time resources should consider this above to decide how to best allocate those resources: how/when/what flavor of swimming should I invest in at different times of the year? The answer is individual and is a function of current swim ability, your personal time constraints, race day goals, etc.
  

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Rich Strauss
Endurance Nation Ironman 2013 and 2014 World Champion TriClub, Div I
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