Long report

I have a rails app that has the ability to export data to excel. The
problem is that the report is very bug, so I need a way to reduce the
waiting time of my clients.

My thought was, lunch some threads... but... is it safe in rails?

You might look into backgrounddrb... you could pass off the generating of the report to it and then have it update a flag in the database that the report is ready or send an email to the user with the url to download it...