There are a few factors that make a difference.

If you live in a really dry climate where it doesn't get excessively hot, the A/C compressor will never suck a whole lot of power. This is because it never sees the high pressures that it would if it was hot and humid.

You'll easily lose a couple of horsepower in any car with the A/C on, even in mild climates. But even in cool weather with extremely high humidity, you're looking at a very big power drain. The air conditioning removes humidity and transfers the heat from the humid air back to the system. The more humidity, the harder the system works, even in cooler temperatures. Therefore, on a cool, humid day, the system might need more power than it would on a warm, dry day. The difference becomes noticeable after working on A/C systems for a while. Most cars are affected, even cars with a powerful engine. Notice that traffic moves a lot slower when it's really hot outside. I attribute a lot of it to A/C draining the useable power from getting to the wheels.

While you might not notice much of a difference when you are in the powerband, say around 3500rpm, that isn't really practical for day to day driving. Try driving an R134a 240 with a 5-speed, climbing a moderate hill in 5th gear at 60mph on a hot day. Better move to the truck lane. Kick down to 4th and you might stand a chance. There is one hill in the near vicinity where I'd just kill the A/C to make it up it at a good speed.

Also, with an R134a system, the pressures are higher. That translates into more power being used by the compressor. Compare the feel between cool, dry days and hot humid days and it's very noticeable.
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chris herbst, near chicago

I must agree with Aye on this one, with a stick you might feel it kicking on and off but it shouldnt cause to much drain, you can always down shift and raise your rpm's easily with a stick, i know my automatic when it kicks on and off i can feel it, and it does take and extra few seconds from normall to reach a determend speed, if its a huge problem, and you just want that extra 2-3 hp back when its on you can try to advance your timeing alittle i have found it will help but only a little, i wouldnt expect much, though if you dont know how to set your timeing now dont go in there and mess with it.

It's possible your compressor is seizing up. If so, it could be sucking up a lot of power. However, it will lose the belt most likely before it seizes, so at least you'll know when you see the smoke and hear the squeal.
It really shouldn't consume so much power that the car feels weak. Maybe 2-3 HP at most. SOmething's wrong here. MAybe the car's got a significant vacuum leak or something robbing power, so it's down on power anyway. A stickshift car ought to feel pretty strong. Check your air hoses for leaks and make sure the timing is set right.
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Rob Bareiss, New London CT ::: "Roterande Fläkt Och Drivremmar!"

Sounds familiar. Whenever hot weather arrives and the A/C goes on I find myself wishing for a turbo. Remember, the NA B230F only develops 114hp, and that's at 5300rpm or so. Put 3100lbs of car on that, plus an A/C compressor, and you don't have a lot of oomph left over! Putting in a Unitek Phase 1 camshaft might help, but now we're into the realm of the theoretical.

jeff daigle
'89 745GL 142,000mi