As my party enters 9th level, buffs & nerfs have started to become a problem. Someone casts Bless, someone casts Haste, and the game takes a pause while we try to remember what type of bonuses those are and will they stack. And then a Prayer gets cast and we start all over. And then someone attacks, and it's "what was my plus to hit and damage again?". And Player #1, with his magical weapon, has to calculate all those bonuses again, differently, because his magic weapon stacks differently. And Player #2 has a cloak so all his numbers are different.
How do folks deal with that?
Tell all your players to basically calculate the stats for the standard buffs already ahead of time, and they then have to keep track of it themselves.
Some people in our group use table tents to indicate bonuses (such as bard performances).
I use index cards and pass them out with the bonuses.
Buff cards and condition summary cards.
Paizo publishes a Buff Deck and a Condition Deck, or you can make your own.
If you make your own, be sure to make enough of each type for each player.
I've heard of people having great success with notecards. Make cards for common spells or abilities, such as "Bless / +1 Attack / Morale" and place them in the middle of the table (or near the appropriate player) when cast. You can also place tokens or coins on them to track duration. When they are all in front of you the basic addition and subtraction really shouldn't be a problem.
You might also want to encourage players to calculate these things before they roll. It will give them something to do between turns, avoiding delays and hopefully preventing the scourge of "Smartphone distraction" that so plagues the world these days.
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For my wife and I with our Oracle and Cleric characters, we use 3x5 note cards with the most necessary buff/debuff information on it, that we then put on the table near the combat, along with a die for counting down rounds/duration of the buff/debuff.
So, for like Bless with my level 1 PFS Cleric, I put down a card near the combat that clearly says in dark Sharpie:
Bless
MORALE Bonus - 1 min
+1 to attack roll
+1 to save vs. fear
I then put a d10 on the corner of the card to tick down the 10 rounds (60 seconds) that it lasts.
The Bard in our group liked this so much that he borrowed a few blank cards and my sharpie to do the same thing with his buffs. Soon, we knew exactly what stacked and what didn't, and players at the table just had to look at each card in play and add the relevant bonuses to their particular roll.
Despite low levels our party cleric buffs often and between that and enemy witch de-buffs it's gotten pretty weird. She uses a few index cards with a die listing off number of rounds. Every round we all just count in whatever bonuses we're still getting and she ticks down the rounds.