I just finished distraction 3 days ago and I reached 8 cm.
I started feeling tight around 4cm so you are feeling it a bit earlier than I am.
There are a couple of factors impacting how tight I got:
1) How much time you invest in stretching
2) Your weight and body type. Generally the thicker you are, and the more leg muscle mass you have, the harder it is fight off the tightness. I had a relative advantage here because I had fairly thin, non muscular legs going into this.
3) Your body position throughout the day. The more your knees are bent during the day (e.g. sitting) and sleeping (e.g. with your knees bent) the tighter you will get, specifically in your IT band. It's best to keep your legs straight as much as possible.
By far the most important thing is 1), how much time you invest in stretching. I've met many LLers during my journey and the main factor in those who successfully hit their CM goal and those who didn't was stretching. The people who hit their goals had a robust stretching regimen and those that failed and tapped out at around 5 to 6 cm were lazy and didn't stretch at all.
I personally stretched 4 to 6x a day. I did my op with Paley which include 1x daily PT, so I would do 3 to 5 stretches at home. Each stretch was 30 to 45 min long and was comprehensive stretches across quads, hamstrings, hips, and groin. I think this dedication to stretching was key to me hitting 8 CM. Even with this amount of time and investment in stretching, I still got tight and had to rate limit my distraction from 1mm to .75mm a few days.
The other option you have, is if you have the time, slow down your rate of distraction from 1MM a day to 0.75mm or even 0.5 mm .
Also, sleep with your legs straight. When you walk, don't focus on speed, focus on form. Narrow your legs, walk with long strides, this helps with stretching your IT bands.
Good luck man !