Pre filtering isn't a huge problem out west, where the water tends to be a little less gunky than what I found while living in TN/KY/GA, so if you need a prefilter, a cotton bandana seems to work well enough to stop the silt and bugs...algae and moss are more an eastern problem in my experience.
There are a few logistical problems to a gravity filter though:
Many water sources are shallow, submerging a bladder to fill can be a difficult if not impossible proposition.
If you use a platypus- a hoser has a small opening and is hard to fill by 'drowning' but the small opening makes for easy covering with cloth to filter out silt and bugs.
a BigZip is easier to fill, but impossible to pre-filter with a bandana when submerging.
A camelbak is a happy medium between the two platy's, but tastes like plastic, and the hose points upward, putting stress on the connection when used as a gravity filter. Also all the ones I have are opaque plastic, which I dislike for some reason.
The best solution is possibly to have a small cup to skim water and dump into the bladder, through a prefilter. This is slow though, filling 3+ L with a 12oz cup can take time. Probably not more time than pumping though.
Once the bladder is full, I've found that the flow rate through the Sawyer filter is roughly equivalent to a normal, unfiltered drinking bladder hose. Pretty acceptable, considering I'm literally doing zero work just holding a container under the hose while the bladder is held up by a tree, trekking pole, or other person.
Here's a pic of my setup. Post filter has a choice of on/off toggle, or drinking mouthpiece. Not pictured is the bandana for filtering, and a cup. I've never found a need for a 'clean' bag. Just hold the 'dirty' water until you run it though the filter into another container, or suck it though the filter by mouth.
Edit: total weight of pictured items is 9 oz.