How does an index speed up insertion in a table How does index speed up insertion in a table?

I have an idea how it happens for retrieving the rows and selection of data, but how does it helps in insertion?

CREATE TABLE  persons(
id NUMBER ,
first_name VARCHAR2(50) NOT NULL,
PRIMARY KEY(id)
);


and if create an index on first_name

create index name_idx on persons(first_name) 


how does it speed up the query below?

INSERT INTO suppliers (id,first_name) VALUES (01, 'SAI');
INSERT INTO suppliers (id,first_name) VALUES (02, 'CHRI');
INSERT INTO suppliers (id,first_name) VALUES (03, 'MASSEY');
INSERT INTO suppliers (id,first_name) VALUES (04, 'FREDDIE');
INSERT INTO suppliers (id,first_name) VALUES (05, 'ROGER');
An index like that with an INSERT like this only makes the INSERT slower.

An index could help if you performed an INSERT .. SELECT .. FROM ... and the index would speed up the SELECT part.