Other than the line you referenced I skimmed the book for improvised and found out these parts...
Blood sorcery page 13:
Rather than enumerate every possible ritual or miracle,
this system breaks the Ritual Disciplines down into
Themes that are then combined to produce new effects
on the fly—a Sanctified master of the Destruction Theme
may attempt any miracle he can think of using that
Theme, without the need to buy them separately. There
is still a reason to buy rituals with Experience, however.
Rituals the practitioner “knows” are easier to cast and
more potent than improvised ones.
Learned rituals are easier to cast, and more potent. System is explained later on.
The dice pool used varies by Discipline and by whether or
not the ritual is already known to the ritualist. Improvised
rituals always use the same Attribute + Skill combination,
adding the character’s dots in the Theme used. Learned
rituals, however, use Attributes and Skills according to the
following table, added to the Theme in use.
Improved and Learned rituals uses different skill sets. (not an advantage for learned rituals thou)
Success: The ritual succeeds and takes place as
described by the player. Excess successes are lost for
improvised rituals, but they may be assigned to the
ritual factor or factors of the player’s choice for learned
Excess successes are lost for improvised
rituals. (so learned ones can be more potent if you have excess success)
Example: An Acolyte ritualist is performing a rite to
immunize her coterie from the effects of Vinculum. This is a
18 Blood Sorcery
Protection •••• effect, so the initial target number of successes
is four. Making the effect last until the next sunrise adds three
successes to the target, and affecting all three members of the
coterie adds another two. The final target number is nine. She
rolls Presence + Stealth three times, and gains ten successes.
Because the ritualist knows the rite, rather than improvising it
from her knowledge of Protection, her player may then spend the
extra success. She chooses to increase the duration to a month.
There are minimal differences between improvised and learned and I cannot see anything proves Improvised rituals are slower or Learned rituals are easier to cast parts.