Re: How to calculate the cumulative value without using the CUMULATE function
You're right that RANKCUMULATE does have performance impact. It is a function that has to run single-threaded, most other Anaplan functions can the benefit of splitting calculations over multiple parallel CPU threads to improve speed; this function doesn't, neither does RANK or ISFIRSTOCCURENCE.
CUMULATE when used over a list will also run single-threaded and this may have been the concerns of @taku.s in the original post. CUMULATE over time does not have that problem, so those solutions here will perform better (than using with a list).
However, I think you missed Rob's point that a if you just need the cumulative total over that list, use a summary. Another similar alternative would be to change the boolean to list formatted and have a list with 2 items, "TRUE" and "FALSE"; you can then do a SUM...