As a matter of interest, could this be tied into this Mark Merlin story somehow?: www.arche-arc.blogspot.com/2017/04/mythcomics-threat-of-horrible-hex-house.html

It seems Mark Merlin faced off against three seventeenth century darkmages in New England in "Threat of the Horrible Hex" House of Secrets 64 (January-February 1964). These involved Spinster Toten (a witch), Black Moon (a Native American medicine man) and Peter Stalb, who has learnt African and Asian darkmagic, including voudoun.

The synopsis for the story is this: Merlin and his companions, Elsa and his feline familiar Memakata, travel to Halzburg, Pennsylvania, a Pennsylvania Dutch settlement whose inhabitants belonged to the Dubenite sect (akin to the Amish). Elsa spies a hex sign on a local barn and Merlin relates the events of 1690, when one of the town founders, Mayor Josef Von Helz, encountered a three-way satanic pact between Spinster Toten, Black Moon and Peter Stalb. He defeats them, but then notes three indelible hex symbols for each darkmage are etched on his barn. Unable to break the curse, as well as destroying the town in the process, it develops that each successive generation of the Von Halz family has vanished on their twenty fifth birthday. Henrietta Von Halz, Josef's latest descendant, isn't keen on following them and asks Merlin and his companions for their supernatural help. Merlin takes the curse onto himself and the three darkmages attack- Black Moon unleashes hurricane winds, Spinster Toten causes a crack in the earth to appear, which Merlin levitates out of, and Stalb uses a voodoo doll to paralyse Merlin's limbs. However, Merlin has noticed the barn's three hex symbols heat up just before the darkmages attack, and that there is a fourth hex symbol. Merlin works out that the fourth symbol was etched by Josef Von Halz to provide a countersign to the darkmages, which he activates, imprisoning them once more:

Source: leagueofcomicgeeks.com/comic/9033182/house-of-secrets-64

One wonders if these three necromancers ever got out of their imprisonment afterward...?

As for Lady Jane, perhaps she was (a) born with cerebral palsy (b) has difficulties forming human words due to her wooden form and dissimilar morphology (c) is not from an anglophone society originally and had to learn English as a second language, so she's hesitant about whether she's got the correct vocabulary, pronunciation or grammar when she speaks* (*which would tie in nicely if you wanted to develop the Mark Merlin/Halzburg angle within your story- what if she's of German derivation herself?)

She is English. Apparently, most of the manifestations of the Green speak in this halting manner.

or maybe their vocal cords are made of wood? They may actually form words by changing the size of a resonator chamber, which takes longer to do with muscle fibres that are made of vegetation, and so they have to speak more slowly? And perhaps their voices are more on the bass end of the range?

Personally I would still rather read their words without a bunch of ellipses to show that they are speaking slowly, and instead have the author remind us periodically that the speaker is talking slowly in very deep tones. But this is only a personal preference - I can see Doc's point, too, of writing the dialog and then inserting ellipses afterwards, so the reader gets a constant reminder.