from mysql 5.7 to mysql 8.4

[Diana B.]

I have a performance problem following a migration from MYSQL 5.7.16 to MYSQL 8.4.0.
My sql 8.4.0 is installed on a windows server 2022 and the previous version 5.7 was also on windows.
The database is used by a web application that has had a sharp drop in performance. I would like to point out that the system is not yet in production and the only person who is using the database is me.
The web application is run by 32-bit IIS and uses the “MySQL ODBC 8.0 Ansi Driver” drive.

I report the tests performed and anomalies identified
1. Restarted mysql service to start from an efficient base
2. I run 1 fairly complex selection query directly from workbench: execution time about 13 seconds. It can happen because it is the first time it is run.
3. I re-run the same query in workbench: execution time: 0.47 seconds.
4. I navigate the web app and I already notice a slowness
5. I re-run the same query in workbench: execution time: 0.47 seconds.
6. I navigate to a specific page that runs a select query similar to the one executed in point 2
7. I re-run the same query in workbench: execution time: 12 seconds. This query that had a time of 0.47 seconds has increased to 12 seconds and its execution times do not decrease until I restart the mysql service

I checked with “process list” if there were any locks, but nothing.

I also report the value of some configured variables:
innodb_buffer_pool_size: 4294967296
innodb_log_buffer_size: 16777216
innodb_flush_log_at_trx_commit: 1
innodb_buffer_pool_bytes_data: 4147019776

Monitoring the server status with mysql workbench on MySql 8.4 I see "key efficient 0%", while on Mysql 5.7 I see "key efficient 100%". Why? I migrated the same keys. Maybe this is the problem?
Can you help me understand how to solve these slowness problems?