It's best to describe the symptoms of a problem before prematurely concluding the underlying cause.
If that Mac's fan is objectionably loud and the Mac is hot and running slowly, the first thing to do is review About fans and fan noise in your Apple product - Apple Support. In it, you will eventually come across this passage from Keep your Mac laptop within acceptable operating temperatures - Apple Support:
"If your Mac laptop gets warm even when it isn’t doing tasks that require intensive calculations, or if it has fans that run for a long time, use Activity Monitor to check CPU activity. Look for malfunctioning processes that can put a significant load on the CPU and check for unexpected fan use. If issues persist, close and open the lid on your laptop, or restart your computer. ..."
Post a screenshot of Activity Monitor if you wish. Use the Energy pane, or CPU sorted by %CPU in descending order to see which apps or processes are using the most energy or placing the greatest demands on your Mac's CPU, respectively. To show the most relevant information I suggest selecting "Active Processes" from Activity Monitor's View menu.
Other thoughts addressed in the above Apple Support documents but are not emphasized enough IMO: never use cases, covers, or "skins" on a Mac.
If nothing helps please write back for additional suggestions.
