Which version of D&D are you running? And what level are the PCs?
If it's 0-1-2e then the simple encounters, particularly at low level, shouldn't take long at all (though for the love of your sanity avoid RAW 1e initiative like the plague!); and for some of them you might not even need a detailed map. At about 4th level or below in 1e I can usually get through several simple encounters in a session, plus some intervening story, scouting, information gathering, and in-character arguments. As the PCs get higher in level, however, even the most basic of encounters can get bogged down in a hurry - my PCs average about 8th-9th level these days and even a simple battle can take half the night.
If it's 3e or 4e you're a bit more up against it at any PC level, as both those kinda demand proper battlemaps and that time be spent on tactics and abilities etc. even in a simple combat. And 4e comes with the added time-sink of higher PC and monster hit points, unless you use a lot of minions...though some of this can be mitigated by skill challenges taking the place of time-consuming exploration and info gathering if you so desire.
Itf it's 5e - well, the advertising told us it could play quick, or be made to, if that's what you want. Reports from posters in here, however, would tend to suggest otherwise; and that's all I can go on as, though I've read the core books, I've yet to play it myself other than a couple of convention games (which are a completely different animal than ongoing home campaign games).
That said, regardless of edition I've a few suggestions that may or may not be practical for you:
- Run longer sessions. I've learned over the years that it generally takes the players an hour or two - sometimes more - to get the socializing and out-of-game chatter out of their systems, plus another hour or so if it's been two weeks instead of one since they last saw each other. Thus, with a 4-hour session you're often just nicely getting started when it's time to pack it in; but with a 6-hour session you'll get those extra couple of hours of good play in.
- Run back-to-back sessions. By this I mean run the same group two nights in a row; or a night and then the following afternoon. The first night will be the usual; but if my own experience is anything to go by the second night will be pure gold - they got all their socializing done the night before and they (and you) haven't had time to forget everything that's going on in the game.
We're playing 5e. No matter the level, the need to run trivial speed bump encounters is a default assumption of the game.
Running longer sessions or two weeks in a row just can't happen due to our schedules, though I'd love it.