Mysql select with WHERE (a, b) in (('str1', 'str2'), ...., ('str6000', 'str6000')) taking 30 seconds to execute

Question

I have table of the following structure:

create table my_table
(
    id          int auto_increment
        primary key,
    name        varchar(255)                             null,
    vendor_name varchar(250) default ''                  not null,
    constraint id
        unique (id, name, vendor_name),
)
create index name
    on my_table (name);

create index vendor_name
    on my_table (vendor_name);

create index tuple_index
    on my_table (name, vendor_name);

It currently has about 2 000 000 rows.

The select I run is:

SELECT *
FROM my_table
WHERE (name, vendor_name) in ((str1, str2), (str3, str4), ..., (str6000, str6001))

The run is taking about 30 seconds to return about 300 rows (only that many matched and it's fine).

I have an index on id (default one), index on name, index on vendor_name, and a combined index on name,vendor_name.

Is it normal for "selects" on string columns to take such a long time?

I tried to EXPLAIN the query but didn't get much from it:

| id | select_type | table    | type | possible_keys                | key | key_len | ref | rows    | Extra       |
| -- | ----------- | -------- | ---- | ---------------------------- | --- | ------- | --- | ------- | ----------- |
|  1 | SIMPLE      | my_table | ALL  | name,vendor_name,tuple_index |     |         |     | 1947772 | Using where |