I set up a skeleton mob spawner with all usual thing for that version and tested it in a nother world and same thing happend where the mob spawner does`t spawn fast and i lit up the caves close to the spawner because i thougt that was the issue but it didn`t fix the problem.

In a previous version i did the same thing and the spawner spawned mobs normaly.

I folowed this design blelow and still works

https://www.youtube.com/watch?v=dhW6DYvwsxc