TL;DR: I'm like 90% sure there is no 'speed /-5' going on. What happens is your units have a 0-5% random starting CR% at the beginning of each battle.

See patch notes 2/20. "Ctrl F" then type "Combat Readiness" you'll see the following line:

At the start of battle, the Combat Readiness variation that can be gained according to the speed stat of a character has been reduced from 10% to 5%.

There's some ambiguity about "according to the speed stat of the character", but I honestly think this is just a really poorly placed 'definition' of what speed does, like "Hey, this is what speed does - it increases combat readiness".

Don't believe me?

Go into the training grounds with any unit from your journal. Click on the little hourglass figure on the top left, notice the sandbags - which have the same stats and their CR never increases no matter how long you're in here - have a random placement of CR from 0% to 5% (mine have two at 4% and one at 0% - this never changes for me (and it won't for you) because you can't 'complete' this stage, so the 'seed' it's saved to will always be the same, unless they update the training room and reset the seeds for everyone, I guess).

• See pic.

• Because this may happen: If you see a value above 5% it might be because you started the game before the change and it's keeping your pre-change 'seed'. I'm not sure if this can happen, only anticipating it happening and trying to get ahead of it.

What does this mean?

It means that the more speed two respective characters have, the wider their respective 'first turns' get in terms of speed differences. Two units - one at 100 speed and one at 106 speed - will never mix their turns up, while two units at 200 and 206 speed will, on occasion, see the 200 speed unit go first. When units start pushing 250 speed this gets even worse. See table:

Note I have rounded down to the nearest whole number, and in the cases where no rounding is necessary (any multiple of 20), I have reduced the speed by 1 "just in case".

"Duh, I already knew this.

A lot of people don't. Most of this sub uses the /-5 speed explanation. But awesome for you! :)

"Hey I have two units that don't fit this table and they never mix their turns."

It's likely still very close to this table and might be due to speed rounding. I actually suspect speed has decimals that aren't shown (and that this is why 5 speed on an ilvl 85 is so rare - my guess is it's actually 4.5 speed rounded up to 5). You can see some discrepancy in speed values if you try to count your speed on your gear and then see what it displays on your character. You could potentially have a swing of 1 speed between two units due to rounding (217.5 displaying as 218 and 206.4 displaying as 206, a difference of 12 is displayed when in fact the actual difference is 11.1 =~11).

It could also be because of my "-1 speed on a multiple of 20" thing.

Also note the case where ONE unit gets 0% and the OTHER gets 5% is actually pretty rare - 1 in 36. This is even rarer if speed has decimal values, and exponentially more rare if the CR% has undisplayed decimal values as well.

Or I could absolutely be blatantly wrong, in which case please discuss below the cases that clearly don't fall under my table, and we can get to the nitty-gritty of what's going on. Keep discussion civil, please. :)

• It could be the "according to unit's speed" thing - where lower speed values have a higher variance? Not sure.