working on speed in off season or few months before IM

I hear where your coming from with this, there is a lot of reading material with conflicting views. My opinion after sifting through everything is this. I you are looking to make great gains in your speed, then your going to need more than a couple of months prior to your event. As you approach your even you should be focusing on race specific training, incorporating shorter, faster sessions.

As I understand it, the ideal time for iccreasing speed is the off season, providing you already have a reasonable endurance base from the previouse year. Focus on speed and strength with shorter, faster runs and rides, including sprint work, and just maintain your longer distance workouts, perhaps once a week. As the season moves forwards, increas the endurance.

Off-season tends to be the better time to put in bigger focus blocks for a specific sport. That's one way to work on it.

Another is a perception thing, in understanding where speed comes from. Off-season is not interrupted by tapers or recoveries for events, so you can put in a good amount of volume if you like. This is actually where speed comes from, by building up a lot of aerobic capacity through a bigger training load. The specifics of what consists of a 'training load' can vary. The speed workouts closer to the race take that aerobic capacity and get you more used to working at an effort & duration appropriate for the race you'll be doing.