difference between a unique constraint and a unique index ???

In order to get like queries to use an index with database initialized
with a UTF-8 character set I added a unique index to a table with a
varchar_pattern_ops

This table already had a unique constraint on the column so I dropped
the unique constraint.

I can't give exact measurements however this caused my application to
slow down considerably.

The only thing I can figure is that the varchar_pattern_ops operator
is significantly slower ???

Is there some other piece of the puzzle to fill in ?

Dave