My avaraged speed unit is between 470 and 500 speed, so I prefer helpers to be around that speed as well, to keep the speed of the whole group relatively even for those maps that require it.
For my regular unit, it doesn't matter much, the main squad is 887 speed, while the second is 742 speed, so they're usually leading the other squads anyway, and generally very capable of clearing an entire map and some alternative routes. My second squad might drop to 590 speed soon though, in which case a fast helper might become useful.

500-600 is best. If I want a 1k speed monster team I'll just use my own.

600-700 as thats my teams speed. But I sometimes will select faster or slower team if map requires me to do so.

I like the 1K+ speed teams

I normally look for 5-600 as it matches my own squad

500 to 700 range is what i prefer depending on the map

500-600 seems the optimal speed. I want the helper to be around the same speed as my team or in some maps slightly faster.

600-700 works for me since my squads are all 600-670 speed.