hendacasyllable vs. iambic pentameter

The 11-syllable line is basically this:

Counting from the back, syllable 10 will be accented.  That's what defines the line as an 11.

Therefore 9 won't be. (Stress clashes are rare in this position.)

There are two main patterns:  accents on 4, 8, 10  and ?, 6, 19.

Since there is an accent on either 6 or 8, you won't find one on 7.  The same thing goes for 5.  

So the sound of the line is determined by its characteristic 6- or 7-syllable cadence at the end.

Syllables 1 through 3 are up for grabs, then. If there is an accent on 6, you will find on on 1, 2, or 3.  With accent on 2, you have equal space between the three accents in the line.

"El dulce lamentar de dos pastores."

So, in comparison to the English iambic pentameter we find:  it is the same length, since 11 is the same as 10 (the last syllable being the unaccented "feminine" ending).

It is more iambic than anything else, since only 1 and 3 get very many stresses.

There are fewer stress clashes in Spanish:

"Siempre la claridad viene del cielo."

They are possible but easy to avoid because there are more spaces between the accents, with more polysyllabic words in Spanish. Good poets in English will mix in polysyllabic words, even one in a line make it much less like Pope's "ten low words" in "one dull line."

The other main difference is in the "melodic" hendecayllable, with accent on 3. You don't find that in English very much.