Slow to respond. The fan is constantly running

See if you can perform the following two tasks to address your concern-

1. Reset SMC How to reset the System Management Controller (SMC) on your Mac - Apple Support If this does not help the perform step 2.
2. Reset NVRAM, PRAM Reset NVRAM or PRAM on your Mac - Apple Support

If none of the above works then have your Macbook examined at an Apple Store.

Axel F.

Another link to article with the word 'fan' inside title and that may offer more clues:

• See how apps affect Mac performance, battery runtime, temperature, and fan activity - Apple Support

https://support.apple.com/en-us/HT203184

Note additional links in above toward bottom of article's contents, with more ideas.

Read About fans and fan noise in your Apple product. A MacBook Air exhaust fan is in constant rotation unless the Mac is sleeping or shut down.

Please determine if the same problems occur in "Safe Mode": Use safe mode to isolate issues with your Mac.