The optimization process and recommendations:

1. Avoid Selecting Unnecessary Columns (query line: 8): Avoid selecting all columns with the '*' wildcard, unless you intend to use them all. Selecting redundant columns may result in unnecessary performance degradation.
2. Create Optimal Indexes (modified query below): The recommended indexes are an integral part of this optimization effort and should be created before testing the execution duration of the optimized query.

Optimal indexes for this query:

CREATE INDEX devices_idx_customer_id ON "devices" ("customer_id");
CREATE INDEX sample_data_idx_time ON "sample_data" ("time");
CREATE INDEX sample_data_idx_device_id_time ON "sample_data" ("device_id","time");
CREATE INDEX sample_data_idx_sample ON "sample_data" ("sample" desc);

The optimized query:

WITH periods AS (SELECT
        time.start AS st,
        time.start + (INTERVAL '1 year' / 100) AS en 
    FROM
        generate_series(now() - INTERVAL '1 year',
        now(),
        INTERVAL '1 year' / 100) AS time(start)) SELECT
        s.* 
    FROM
        sample_data s 
    JOIN
        periods 
            ON s.time BETWEEN periods.st AND periods.en 
    JOIN
        devices d 
            ON d.customer_id = 23 
    WHERE
        s.id = (
            SELECT
                sample_data.id 
            FROM
                sample_data 
            WHERE
                sample_data.device_id = d.id 
                AND sample_data.time BETWEEN periods.st AND periods.en 
            ORDER BY
                sample_data.sample ASC LIMIT 1
        ) 
        OR s.id = (
            SELECT
                sample_data.id 
            FROM
                sample_data 
            WHERE
                sample_data.device_id = d.id 
                AND sample_data.time BETWEEN periods.st AND periods.en 
            ORDER BY
                sample_data.sample DESC LIMIT 1
        )