Sometimes the error.log file can grow very large due to many PHP errors being written to it.
This could slow down your site because writing to a large error.log file takes more time.
It might also use up your disk space quota if left to grow without intervention.
The CPanel File Manager will not open error.log files over 1 MB in size. (you can delete the file and a new error.log file will automatically be created)
Preventing the errors from being written to the error.log file can be a temporary workaround to keep the site loading fast until there is time to get to the root of the problem.
In your CPanel go to Software > Select PHP Version.
Click "Switch To PHP Options" (top right).
Set "log-errors" to Off.
Click Save a bottom.
This will prevent PHP errors from being written to error.log files.
Generally it is probably best to leave log_errors off and only turn on when troubleshooting.