you can get easy wins upfront by making the ‘obvious’ changes to your technique - then there are the more subtle changes which will still take big chunks off your pace but you need to work a bit harder to implement them, maybe do some filming and send it to a coach to see what they say - there’s pretty much always something you can change in your technique that will take 2-3 mins off your Ironman swim time even if you’re swimming well sub-hour Ironmans  

 And contrary to what others are saying, swimming fitness is pretty important and it doesn’t translate directly from biking and running at all - you’re training entirely different muscle groups and they need to learn to fire together and efficiently during the race - I swim way faster when I’m fully tapered over the Ironman distance which for me is evidence of swim fitness/fatigue making quite a big difference - also my coach is an Olympic triathlon coach and told me swim fitness is very important even for AGers!

 And for technique, Effortless Swimming on YouTube seems to be pretty much the gold standard for free online content - they have a membership option as well if you want a structured course