MySQL Similar Queries taking longer - mysql

Question

So I have a table about 2GB in size, I run 2 queries and one takes about 200ms and the other takes over 8s, both return '0' which is correct. I added device_id and time_server as indexes and assumed it would make them quicker, it did.. well for one of the queries. So why is there huge difference in time taken to query the same table?
Have I just been unlucky in that one query is running in memory and the other is running from disk as I've hit the limit of innodb_buffer_pool_size?
Why the difference in the row counts from EXPLAIN, if both return a count of 0, I'd have thought it would do a full table scan and the row count would be identical?
Its worth noting that CPU, RAM, Disk I/O etc.. are all fine with nothing obvious that could slow it down. Repeatedly running the queries gives the same results, so its consistent.
Query 1
mysql> SELECT count(*) AS count
FROM mydb.gps
WHERE device_id = 780 AND time_server > '2021-08-03 16:32:48';
+-------+
| count |
+-------+
| 0 |
+-------+
1 row in set (8.20 sec)
Query 2:
mysql> SELECT count(*) AS count
FROM mydb.gps
WHERE device_id = 430 AND time_server > '2021-08-03 16:32:48';
+-------+
| count |
+-------+
| 0 |
+-------+
1 row in set (0.02 sec)
If I run an explain on them:
Query 1
mysql> EXPLAIN
-> SELECT count(*) AS count FROM mydb.gps WHERE device_id = 780 AND time_server > '2021-08-03 16:32:48';
+----+-------------+-------+------------+------+-----------------------+-----------+---------+-------+--------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+-----------------------+-----------+---------+-------+--------+----------+-------------+
| 1 | SIMPLE | gps | NULL | ref | device_id,time_server | device_id | 5 | const | 282416 | 2.12 | Using where |
+----+-------------+-------+------------+------+-----------------------+-----------+---------+-------+--------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
Query 2
mysql> EXPLAIN
-> SELECT count(*) AS count FROM mydb.gps WHERE device_id = 430 AND time_server > '2021-08-03 16:32:48';
+----+-------------+-------+------------+------+-----------------------+-----------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+-----------------------+-----------+---------+-------+------+----------+-------------+
| 1 | SIMPLE | gps | NULL | ref | device_id,time_server | device_id | 5 | const | 2001 | 2.12 | Using where |
+----+-------------+-------+------------+------+-----------------------+-----------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

Answer 1

You have more than 100x more rows with the first device_id, so the query has more rows to scan to check the time_server value. You may be able to improve it by creating a multi-column index:
ALTER TABLE gps ADD INDEX (device_id, time_server);