I have played and DMed both Pathfinder and 5e, here are some quick takeaways one question at a time.
Does 5E's smaller number of modifiers speed play significantly?
Yes, most characters basically need 7 numbers to operate. The game requires you to memorize about 3 more numbers.
- 6 stat modifiers
- 1 proficiency bonus
- the number
10 (base AC)
- the number
8 (base save modifier)
- Concentration Check =>
MAX(10, half-damage) vs CON save
Most numbers in the game are
Stat + Proficiency Bonus or
Stat + Proficiency Bonus + Base. This makes it trivial to re-calculate a number on the fly.
If you compare this to something like calculating "AC" in 3.5/PF, it's night and day. I need HeroLab to keep this all straight for my 11th level Psychic Warrior.
Does 5E's pre-combat buff management take a long time? That is, can players of upper-level characters more quickly pick their spells and effects than in 3.5 or Pathfinder?
"Hey Pathfinders, we're at the last room in the dungeon, this is clearly the BBEG, everybody ready?..."
This is inevitably following by the opening of spell books and ordering of appropriate spells by duration: Resist Energy, then Death Ward, then Protection from Evil, then Bless, then Enlarge Person(s), Bull's Endurance, then finally Haste as the party is ready to charge into the room... at this point the character sheets have scribbles everywhere and each action is followed by "did you remember your bonus from Haste and Bless?".
In 5e the total spell selection is significantly reduced and Concentration really limits the active buff/debuff effects. Bless & Haste and whatever the Bard is doing. Plus, Bardic Inspiration and Bless actually provide a "die" as a bonus, so it's really easy to just hand the person a die and say "you are blessed/inspired" and it's much easier to remember.
The 5e spells are also relatively more valuable. Between "scaling" of spells, less spell slots and flat DCs, your early spell slots do not become "useless" in the way they do in PF. People aren't running around lobbing buffs every combat because most buffs require Concentration, they get fired off one at time. Combat Cantrips are still used at higher levels.
Does 5E's theater-of-the-mind combat style end at-the-table arguments or encourage them when upper-level effects are used?
Really no difference here. I typically use minis for precision, but that's neither faster nor slower between editions.
Other stuff is faster too...
Monsters are much simpler in 5e.
Many high level PF Monsters actually have a selection of Feats on top of their written abilities which can lead to lots of back and forth between books. Some PF monsters are more powerful specifically because of feats like Great Cleave or Improved Bull Rush. But if you're not paying attention, it's really easy to miss this and play the monster wrong.
For example, take the Great Wyrm Red Dragon. It has 16 feats. Many of the feats are "built-in" to things like Saves and Attacks, however a few of them are actually quite relevant and not "built-in".
- Improved Iron Will: re-roll a Will save once per day!
- Cleave: which actually seems irrelevant.
- Quicken Spell: obviously a big deal given the number of spells this thing has.
In 5e most of this complexity is gone. An Ancient Red Dragon is still insanely dangerous but only has about 10 things to keep track of in combat. The Legendary Resistance and Legendary Actions also make it far more reasonable to run against a party because it can actually get 4 actions spread across a turn.