I prefer allies with medium ~550 speed for events and new map. For 0 stamina secret map from gacha events I prefer really fast team with more than 700 speed. For manyu/dragons maps, I don't care.
And for map that needs team with really high speed difference, I had allies with speed from ~450 to >700.

I'm actually a sucker for most of the speed boosting girls, Anthurium and Alpinia in particular, so I tend to pick teams that lean on the faster side as far as helpers go if only because Anthurium is best girl. It tends to work better than slower 400-550 teams if only because most of my teams fall in the 600+ speed ranges.

Most of the time my Teams have ~500-550 Speed, so I like to have a Helper with the same Speed or slightly faster (because play last). So I answered 500-600.
Note sometimes I prefer 400-500, so overall, I would say 400-600.

600 is my butterzone; not too fast to drastically outspeed my squads when trying to 3-medal clear a map, but fast enough.