The money table from a study of how quickly various languages can convey the same information is shown below.
"LANGUAGE INFORMATION DENSITY-- SYLLABIC RATE --INFORMATION RATE
English 0.91 (± 0.04) 6.19 (± 0.16) 1.08 (± 0.08)
French 0.74 (± 0.04) 7.18 (± 0.12) 0.99 (± 0.09)
German 0.79 (± 0.03) 5.97 (± 0.19) 0.90 (± 0.07)
Italian 0.72 (± 0.04) 6.99 (± 0.23) 0.96 (± 0.10)
Japanese 0.49 (± 0.02) 7.84 (± 0.09) 0.74 (± 0.06)
Mandarin 0.94 (± 0.04) 5.18 (± 0.15) 0.94 (± 0.08)
Spanish 0.63 (± 0.02) 7.82 (± 0.16) 0.98 (± 0.07)
Vietnamese 1 (reference) 5.22 (± 0.08) 1 (reference)
TABLE 1. Cross-language comparison of information density, syllabic rate, and information rate (mean values and 95% confidence intervals). Vietnamese is used as the external reference."
Japanese speakers actually speak more quickly than those of any other language, but the increased speed (similar to Spanish) isn't enough to make up for the increased number of syllables necessary to convey the same information in Japanese relative to many other languages.
German conveyed information the next most slowly. There was no statistically signficant difference at even a one standard deviation level between the other landuages, and German was just 1.5 standard deviations from the mean, which is hardly exceptional in a study looking at eight languages. Japanese, in contrast, was four standard deviations below the norm in information transmission rate.
One important factor that distinguished Japanese from the other languages was its exceptionally low inventory of syllables (416 v. runner up 1,191 in Mandarin before accounting for tones v. 7,931 in English, with the most options). In languages other than Japanese, the rate at which people speak and the differences in the number of syllables need to convey information that flows largely from the number of sylllables available in the language are basically sufficient to compensate for each other. But, the amount of information that can be conveyed in a given number of syllables is empirically linked to the number of syllables available in that language, and the lack of phonetic options in Japanese is so great that a fast pace of speech can't overcome it.
This is great news for anyone worried about being able to pronounce Japanese words, as there are fewer syllable sounds to learn. But, it means that Japanese take about 35% longer to convey the same information than its linguistic peers in this simple test.
Perhaps this reality may also explain why the most characteristic of all Japanese poetry forms, the Haiku, is so terse. Efficiency in formulating thoughts matters more if it takes more time to convey them.
Now, it may be that there are other linguistic strategies that are used to address this that a simply loose translation of twenty identical short texts can't convey - for example, maybe conversational Japanese lets more go unspoken. And, some kinds of information in Japanese, for example, the Chinese style numbers use for mathematical calculations (Japanese has quite a few parallel number word systems for smaller numbers), are actually more phonetically compact in Japanese than in any of the Eurpoean languages. But, the outlier is quite interesting and deserves further examination.