Ah - N/M!
I was going down a rabbit hole. I assumed it was the related field that was the issue, so was focusing on that. In fact I just needed the get summary at the key table side.
GetSummary ( PaymentTotal; Month) - OUTGOINGS::PaymentTotal
Not sure how much data you have but this kind of data is usually static when a "period is closed". With that in mind, it may be much better for performance to pre-summarize the data through a script and set the result as static number fields. That will avoid making FM re-calculate the same thing over and over every time the report is run.