No the loose chain is not the reason but since it is loose that means you're not taking very good care of that bike and more important maintenance is required.

Know how to check the engine oil level? Clean or replace the air filter? Check the spark plug(s)?

Always give year, make and model #

Engines do not last forever.

They do ware out and require rebuilds.

A loose chain will not decrease the engine's performance.

Ignorance is the reason. Gitcha some motorcycle edification.

Serial neglect is probably the reason.

No