Answer: Without a primary key or unique constraint, some operations may be slow.

Four different flavors of this message may be produced, flagged as Note, Warning, Caution and Danger:

Note: The table ... has no primary key or unique constraint, but does have a unique index.

This table currently has fewer than 1,000 rows so there may be no performance problems, but if the table grows larger those operations may become extremely slow.

Warning: The table ... has no primary key or unique constraint, but does have a unique index.

This table currently has more than 1,000 rows; performance may not be a problem now but if the table grows larger those operations may become extremely slow.

Caution: The table ... has no primary key or unique constraint, but does have a unique index.

This table has more than 100,000 rows so there is a very real chance those operations will be extremely slow.

Danger: The table ... has no primary key or unique constraint, but does have a unique index.

This table has more than one million rows so there is a very real danger those operations will be extremely slow; think geological time scale :)

See also...
The table ... has no primary key, unique constraint or unique index.