* "Exploring" actions take place on the turn scale (1 turn=10 minutes). When you search a room, it takes a turn. When you engage in combat, it takes 1 turn. (Admittedly AD&D has 1 minute rounds compared to the shorter rounds of Basic, but I hope most combats don't go above 10 rounds...)
I just recently started a monthly 1e AD&D game after having run 3.x D&D, Pathfinder and before that 2e for about 10 years dating back to 1989. I haven't played or run 1e since approximately late 1988 or early 1989. I started the 1e game for a variety of reasons, the most important of which are that I wasn't enjoying the complexity of the newer editions of D&D anymore and with the game only being monthly, I felt like we'd get more accomplished with an old school set of rules.
So far in two 6 hour sessions, I believe we have accomplished more than we did in four of my old bi-weekly 5 hour sessions of Pathfinder or 3.5.
In those two sessions of 12 hours of actual play time, we have managed to have 11 combats and plenty of role-play, character development and exploration. In fact, none of the combats has lasted longer than 20 minutes with most falling into the 10-15 minute range. The characters all started at 1st level and are running through the Temple of Elemental Evil. They have run around in Hommlet for awhile, and have cleared out the entire upper level of the Moathouse and are working on the lower level now. In approximately half of the combats, I haven't even bothered to set up miniatures. I've only used miniatures to display what the situation looks like in the more complex battle set-ups. Otherwise, any use of miniatures has really only been to illustrate general location of the PCs and the enemies and not for precise and tactical chess-like movement. I've generally done initiative as per the rules in OSRIC, which I find far clearer than AD&D, using 1d6 for each side in the battle. I've found that combat runs extremely quickly, far faster than in 3.x D&D, and this allows us to get back to the good stuff...adventuring, role-playing and character development...that much quicker.
As for level advancement, I haven't found it to be terribly slow. Handing out experience awards pretty much btb, including xp for treasure, has seen the PCs all hit level 2 after two sessions (and the thief has made it to level 3). This of course might change as we move forward but that remains to be seen.
I am greatly enjoying running 1e AD&D again and the players are all loving it.