After weeks/months of preparation, you started your race excited about what the day had in store for you just to end up disappointed hours later with a performance below of what you were expecting or even a DNF (did not finish).
Unfortunately, that's more common than not when racing Ironman because it is such a long day it provides a series of challenges to our physical & mental preparation, our discipline, etc. Still, a bad day can be a great learning opportunity to avoid making the same mistakes in the future and help you better prepare for the next race.
Also, a common mistake I see athletes do after a bad result is to over analyze many potential reasons for the result when in reality, the answer is simpler than that. Below are some of the things I like to consider with the athletes we coach/advise that may help you pinpoint what led you to a subpar performance and fix it.
Training - Did you do the adequate train to race at 'x' power/HR/pace or finish the distance? It's common that the main reason you may have underperformed is simple your body wasn't 100% ready for what you demanded of it on race day. Perhaps you didn't log enough load (volume + specific intensity), long sessions, etc. hence when you pieced a long day together (swim + bike + run) your body simply couldn't sustain the effort.
Right Training - Did you do the specific type of training for the distnace or kind of performance you attempted on race day? That is, if you did NOT spend the bulk of your training at Ironman pace or lower, your body wasn't prepared correctly. Focusing on a shiny high FTP or fast track workouts may be satisfying for the ego, but doesn't necessartily equate to a fast Ironman performance. Those could be included as the "icing on the cake", but you need to bake that cake first!
Pacing - more often than not we tend to overstimate what sort of effort they can sustain for a race. And even if you have your pace dialed in, if you don't account for race day conditions, your "usual" race effort (power/speed) under regular circulstances should be different if the race is hot, humid, windy, etc. In hot/humid days, your power/pace will be lower than normal. If you don't adjust this, you may struggle either during the bike or for sure during the run.
Fueling - A common mistake we experience is not practicing 100% the fueling plan we execute on race day. That is, in training we may be more casual about when we drink/eat what, what do we eat/drink, & how do we eat it. For instance, you may stop at a gas station, chat with friends while eating your drink/food and the ride. Then on race day all of the sudden you are eating on a schedule while riding and your body isn't trained for it. Other could be taking stuff you are not trained for (different products) & not doing a long race simulation in which you consumed the same fueling products for many hours to see how the body would respond.
Pre-race "controllables" - Was your race on a place with different time zone, weather, terrain, altitude, etc.? If so, if you travelled just a few days before the event, your body probably didn't have enough time to adapt to the conditions, or let you get into a proper sleeping cycle. Add to that the stress of traveling, the inability of many to rest when not sleeping on one's bed, etc. all could add up and result on your body feeling "off" on race day.
Specific Preparation - each race offers a set of challenges that you must prepare for. If a race has a hilly bike course and you don't prepare for this, you may struggle. If a race is flat and you don't ride aero all the time, you may struggle. Next are common reasons why each leg of the race may have been below your expectations:
Those are the more common things I would look at first when assessing a subpar performance for an athlete. As you go through the list more often than not, the answer will me rather simple and doing this self-assessment will help you stay accountable, learn from your mistakes and keep developing your experience within the sport!
Learn from your experience and don't walk away thinking you failed. After all, it's will only be a failure if you didn't learn from it and work on it so you can get things right in the future!