@dizmox While reading a lot will help get you reading faster eventually, there are also a couple exercises you can do that can help you break a few bad reading habits. I took these from some speed reading guides (I wouldn't actually suggest learning to speed read. To me it seems a little shady.) but some of the beginner techniques are legit.
1. Avoid stopping at every word to read. Take your finger and slide it under the sentence you're reading and let your eyes follow your finger. This should be a very smooth motion, stopping only at the end of lines. (Also a bit awkward with Japanese books, I know.)
2. Avoid backtracking as much as possible. Of course you should go back if you haven't understood something (especially if the sentence was constructed with nothing but words you know), but this is obviously something you should try to avoid because it adds more time to reading.
3. Take a few minutes each day to read way faster than you should. Try as hard as you can to understand but don't worry if you don't. You won't make much progress if you don't push yourself.
Now to get a little meta.
Personally, I've found that my slow reading is mostly due to concentration issues. If I'm not 100% invested in what I'm reading, my reading speed is going to be slower because I'm going to have trouble understanding everything. In other words, my concentration directly influences the amount of information my brain can take in per minute so more concentration equals faster reading. If this is true, than I think it would be fair to say that the only way I can read faster while understanding what I'm reading is by upping my concentration. Good concentration is needed for making exercises 2 and 3 useful.
Lately I've been finding ways to keep myself more concentrated while reading. For example visualizing what I read, every last detail (if I don't understand 100% what I'm reading or if I'm drifting off, the picture won't be complete), and I also take small breaks between chapters to let my mind rest. I also sometimes use the breaks to review what I've just read in the language I'm reading as oral practice.
I feel that, after passion, everything in language learning comes down to concentration and paying close attention, so becoming a good, quick reader is also a good way to practice becoming a good language learner. Too bad I'm still a bad, slow reader.