As far as I've been able to find out, it just seems that way. Although I have read statements that Spanish speakers use more syllables per minute than do English speakers, I have searched in vain for any reliable studies to back up that belief. Even if we knew that Spanish speakers in general used more syllables per minute, that might not mean a whole lot, because Spanish syllables tend to be shorter than English ones.
In any case, it is difficult to make comparisons. The rate of speech can very enormously even among individual speakers. I remember watching the Mexican president (then Vicente Fox) give a formal speech, and he spoke at a rate that made him quite easy to be understood. But in an interview later that day, he spoke more rapidly, and I assume that if he were in an animated conversation he would speak at a rate that would make it difficult for non-native speakers to comprehend him.
Pay attention to your own rate of speech. In a given day you may speak quite deliberately at times with careful enunciation, while at other times you may speak "a mile a minute." The same is true for Spanish speakers.
Whatever the differences are, probably the reason it seems like Spanish is so much faster is because you don't know the language. Since you know English well, you don't have to hear every single sound in every single word to know what is said, because your mind is able to fill in the gaps and to determine where one word ends and the next begins. But until you know another language well, you don't have that ability with it.
It also seems to be true that the process of elision the omission of sounds as words run together is more extensive in Spanish than it is in English (although perhaps not as extensive as in French). In Spanish, for example, a phrase such as ella ha hablado (meaning "she has spoken") typically will end up sounding like ellablado, meaning the distinct sound of an entire word (ha) plus part of another word are gone. Also, most Spanish consonants (other than the r) can seem indistinct to the ear accustomed to English, making understanding a bit more difficult.
I don't know of any fixes for the problem, except that practice makes perfect (or if not perfect, better). As you learn Spanish, try to listen for Spanish phrases rather than individual words, and perhaps that will speed up the process of understanding.
Addendum: The following letter received after the initial publication of this article raises some interesting points. One of them, about the different formation of syllables in the two languages, make sense, so I'm adding the letter here:
Somewhere I read the results of a study that concluded Spanish is spoken more rapidly than English. The reason is the typical Spanish syllable is open (meaning consonant-vowel) while in English the typical syllable is closed (consonant-vowel-consonant). Words with more than one syllable in English tend to have two dissimilar consonants together requiring a slowing of speech to sound both of them.
We natural English speakers get to be pretty adept at sounding two consonants together, but it is tough for a natural Spanish speaker to do. In Spanish when two consonants are together the natural speaker often inserts an extra (unwritten and soft) vowel sound between them. For example in the Spanish word AGRUPADO, you can hear it pronounced AGuRUPADO. The extra u is short and soft, but separates the consonants. Natural English speakers have no problem sounding "GR" without inserting an extra vowel, but we do it at a slightly slower rate.
Your comments about Vicente Fox are interesting. I have found political figures usually speak so clearly that I can understand them better than the general Spanish speaking public. This is especially true when they are giving addresses. Although I seldom liked what he said, I used to enjoy listening to Fidel Castro because he was so easy to understand. These days his voice has a senile quality that interferes with clarity somewhat. Most ministers have the same clear speech as political leaders, and thus religious services are good places to practice your Spanish listening skills if you are a learner.