Mail in ballots is PA cannot be legally counted before Election Day.

Also, in the past, mail in ballots were often not counted in districts until much later if it was determined that the number of mail in ballots could would not have a statical impact on the election. If a district has 100,000 counted “regular” votes with a 95/5 dem/rep split and there were 1,000 main in ballots to count, there is no need to count them ASAP as if all 1,000 were for one candidate or the other, it would not matter vs 100,000 already counted votes.