Hi, although Forest computes the distribution of items in the "transform vertices" stage, this rendering phase is used also for other Max processes, so it's difficult to know what is causing the slowdoness.
The number of threads used by Forest is automatic, but you can override it using General->CPU threads. Please, try using a lower value (< 10) and check if there is some difference.
Also according to the screenshot, your Forest object scatters more than 1.7 mill. of items. But this value is estimated, the real number of items may be greater.
I've attached a test scene with 4 mill. of teapots. Please, render it and check what is the "transform vertices" time for your computer.