Here is a house rule - house ruling, perhaps is better - we've been using for so long I can't remember when we started.
Standard rules in GURPS Magic say that potions take effect instantly (Magic, p. 213), but don't give much guidance that I can find on how long it takes to get one out, open it, and quaff it.
In GURPS Dungeon Fantasy, potions are a wee bit slower. Adventurers says that "Drinkables take effect a turn after being guzzled" (DF1, p. 29) and further that "Once in hand, it takes a Ready maneuver to open, another to drink." (DF1, p. 28)
So the rules as written say it takes 2 turns to use a drinkable potion, 3 turns for it to take effect (although you can do other stuff that 3rd turn.) Fast-Draw (Potion) only helps you get it in hand faster, and doesn't cut down on those two Ready maneuvers to open and then drink it.
I use much more generous rules.
As we run it, it takes 1-2 turns to use a drinkable potion. First, you Ready it (1 turn, with Ready, instantly with a successful Fast-Draw roll), which gets it out and open for use. Yes, Fast-Draw includes fast-opening - we figure potion bottles are designed for quick opening with a single hand - you're basically thumb-popping the stopper off.
Next, you drink it (1 turn, with Ready) and it takes effect immediately.
This no doubt makes potions more valuable - it's significantly faster, and can take as little as "I take out and drink a potion" to get some effect from it.
Why not use the RAW?
Why bother teaching everyone the longer sequence and confusing them? Plus I like how it makes potions a quick and easy tool to use, which encourages more choice between immediately useful actions. Further, it means potions are purchased even more (more money drain, more need to adventure to replenish it), fights can be tougher (more healing and more potion-buffing means you can fight tougher stuff), and adventures can be that much more challenging. To me, it's a win-win scenario.
Besides, most of us played a lot of Diablo. We all know using a potion just means tapping a key and, bam, you're fully healed. Heh.