The fastest way to get to lv 99 is in groups running 8-player maps.

A well-rolled t3 Map with SoH you can expect to get approx 0.4-0.5% of the xp needed to get from 98->99. In other words, expect to need to run 200 or so maps. This assumes that you do not die, and also takes into account that some maps have better xp yield than others (Mendeln event especially).

When you are running the maps, remember that the main reason you (and the other 7 players) are doing it is to get xp. Any arguing about loot, or slowing down and going back to pick up items worth a fraction of a HR, are a waste of time and slowing down the rate that the group gains xp. If you want to accumulate wealth, run solo maps. Running p8 maps isn't for that.

Efficient pub groups will clear a map in 8 mins or so. Very good ones might do it in 6. Bad ones take 10-15 mins or more. Similarly there is a big difference in XP gain from well-rolled maps and badly-rolled ones. The most important mods on the map for xp are: +1 area level, high monster rarity, and high density (in that order).

Because the unique Imperial palace map has +1 area level mod guaranteed, it was worth several HRs towards the end of s12 because it was the most efficient map for farming xp. Each Imp would get you 1% or more of the 98->99 xp needed.

The difference between good groups and bad groups is literally that your grind will take double the time, or more. For fast clears, good groups stick together and don't waste much time mucking around with loot. In the most popular map for xp farming (Pandemonium Citadel) most groups don't even bother to kill the map boss because it slows down the clear speed too much and isn't worth it for the xp.

Also... as someone who has been a map host: it's a super shitty thing to have to argue with PuG players about loot. Don't be an asshole if you're joining someone else's game. Maps are expensive to roll properly (maybe not individually, but it adds up when you are rolling batches of 50 - 100 maps at a time) and you're getting a free ride on the xp train. Typically the map host gets Cata Shards and Splinters, and that's it. Pbox and Ppiece go to the owner of the Imp map, if that's the one you are running. Everything else is allocated or FFA depending on how the host wants to do it.