If manual focusing is a slow affair, then that leaves auto-focusing as the way to go. But the AF system on the X100 presents its own set of problems.
The X100's multi-area AF system, where you let the X100 determine the focus automatically, is not the best we've seen. It's not as fast or as accurate as the quick AF on cameras like the Panasonic GF2 or the Olympus E-PL2. In fact, it's frustrating; the camera will more often than not select what you don't want rather than what you do want as the focal point. The X100's AF system feels comparable to the AF speeds and accuracy of the first-generation Olympus E-P1 camera without the subsequent updates. To put it simply, the multi-area AF needs work.
The other option is to switch to Area AF mode, where you can manually select the AF point. The idea is to let you manually select focus points so that you can achieve focus faster, but this feature is hampered by its execution. For one, there are a total of 49 selectable focus points. While scrolling through 49 points is fine for still subjects, it feels too many for shooting and changing points on the fly.
While 49 points is admirably comparable to the top-end DSLR cameras out there, the X100's smaller control wheel and directional pad have neither the size nor the sturdiness to keep up with the complexity. You can helpfully adjust the size of the AF frame to be bigger, but it still moves around the screen within the 49 points. In OVF mode the points are reduced to a more manageable 25, but this is done by taking away the 24 points on the outer edges, instead of enlarging the 25 to fill the frame.
To change AF points, you have to press and hold down on the AF button, then scroll through the points using the control wheel or change directions using the d-pad. It's hard to find the AF button located on the left of the LCD, third from down, amongst three other identical buttons with your eye to the lens. Sure, you can use the small ridge below the Play button as a guide, but even that is difficult, and this is where a differentiating feature might have helped; like a raised dot on the AF button. Otherwise, you either take your eye off the screen to find the button or search for it - both of which take time.
The fact that you have to press and hold down on the AF button to make changes also means that changing AF points is a two-handed affair, instead of on some other cameras where you only have to press once to signal your intention to change AF points, then change the point using the d-pad or control wheel, thus freeing your other hand.
It sounds like a bit of work, and it is. The X100 is a camera you have to work with to get what you want, but once your AF point is fixed, the camera usually gets the focus right and in time. In-between multi-area AF and area AF, you're better off shooting in area AF mode, focusing and re-composing if needed.
Unless you're shooting macro, which opens up another quirk you should know.
The minimum focusing distance from the lens is approximately 80cm; anything inside that range and the X100 needs you to manually switch to macro mode in order to focus. Now, 80cm isn't a lot - a friend can be sitting next to you within 80cm and you won't be able to focus on her face if you're not in macro mode.
It's an odd quirk of the X100 - shooting a close but not macro subject in macro mode. It begets a quick familiarity with the macro mode, which is easily turned on by pressing left on the d-pad. If you're using the optical viewfinder, the X100 then switches to the electronic viewfinder for macro shots.
While 80cm is quite short for normal focusing, the X100 can focus on subjects as close as 10cm away from the lens while in macro mode, which lets you get very close to your subject. Be careful though, if you get too close, the X100 might still be able to focus, but you'll get a funky, extreme-background blur and soft effect.
While the X100 has a burst mode to shoot quick, multiple shots, it's not really the mode you want to be in to shoot quick, multiple shots. Once you hit the maximum number of images the X100 can squeeze off in one sequence (according to specs, up to 10 JPEGs and up to 8 RAW or RAW+JPEGs), the camera will lock up for several seconds to save the images. Even the viewfinder will black out to show previews - which means that in the midst of action, you won't be able to shoot anything for many precious moments.
You're better off shooting in Single frame mode, and taking multiple shots as quick as you can. It's a tad slower, but at least your camera won't lock up. The X100 will keep on writing to the card in the background, but you'll still be able to shoot.
The X100's slow write speeds make us feel that the X100 could have benefited from a larger buffer and faster processor, but then again this isn't a sports camera. Instead, it's a camera which forces - or invites - a more considered way of shooting.