Very slow select

I defined a table with two indexed text columns, a and b, of variable length.
This table contains 287195 rows.
SELECT FROM t WHERE a='my string'; works very fast. The same select made on b is very slow (2 or 3 seconds), as if no
indexexists for column b. 
I'd like to know why. Note that b contains very long strings, more than 500 chars. Is there a limit in indexing?
Thanks. Stefano
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Dott. Stefano Bargioni
Biblioteca della Pontificia Universita' della Santa Croce - Roma
<mailto:bargioni@usc.urbe.it>                  <http://www.usc.urbe.it>
Personal web page:    <http://www.usc.urbe.it/html/php.script?bargioni>
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