Remember that I am an Old Coot. A real "Trekker". Not a "Trekkie". We were glued to the old sometimes color TV watching the first series back in 1966. High school.
Navigation is the concern. You do not want to fly through something that might disturb that double warp bubble. Known space lanes allow higher safe Warp Speeds.
Unknown space dictated either slow Warp Speeds or a military grade Star Fleet sensor suite. Sometimes both. Some space could not be attempted at Warp Speed. Not safe.
Voyager was flung clear across the Galaxy. Unknown space. A limited fuel supply. Captain Janeway got her crew, (somewhat) and her ship, (mostly) home because of luck.
Warp One is 1X light speed. Warp 2 is 2X2 light speed. Warp 3 is 3X3X3. Warp 4 is 4X4X4X4 light speed. And so on. The power requirements also jump the same. $Spendy$.
Only military grade warp engines can obtain and maintain high warp speeds. Energy intensive. You also must know exactly where you are going. Navigation. HB of CJ (old coot) Cm.
I think Spock had the trimmed sinister looking beard in the episode; "Dark Mirror", but do not hold me to it.