I am experiencing a lag in gear shifting with my Kia manual petrol. How can this issue be resolved?

From your question, it seems your car’s gear shifts are not as smooth as they were before. Have you changed the gear oil recently? If not, this is likely the most probable cause. It is also possible that, with age, the linkages or cables have worn out. These are relatively easy fixes, and a service centre or any local mechanic should be able to diagnose and resolve the issue.