Error entries in Watchdog duplicated dozen and hundreds of times
The best is to check what these entries are and try to fix them, e.g.:


when the warning/error comes from contrib module, consider searching for this specific error and find out what's the solution (sometimes it may be misconfigured module),
when warnings comes from your custom modules or themes, consider fixing it manually.


If you think you cannot fix the source of the issues, you should introduce caching mechanism to your site such as:


Redis/Memcached which will limit enormous number of inserts into cache tables.
Varnish to cache the front end pages.


or limit the entries by:


limiting watchdog entries to warnings only (ignoring PHP notices) or silence all of them.
disabling dblog (database logging) module completely and switch to syslog logging instead.


See also: How do you improve Drupal performance?