first, 1 would check with the hosting provider – slow server, oversold….
then,
-You may try deleting revisions in database. I tried once and reduced the database to about half. have alook at this guide and see if it helps: https://weblogtoolscollection.com/archives/2010/09/28/deleting-wordpress-revisions/
-You may performance test your site here and fix all the errors:
https://www.webpagetest.org/result/110206_W1_ff224fcb9b0c30bcf742d5307086621f/
– you may performance test your site here as well, Gzip compression etc.:
https://ismyblogworking.com/
-You may try some cache plugins.
– you may try Increasing the memory available to PHP?
-Some plugins can use lots of resources. You may deactivating plugins.
-You may also want to discuss with your hosting provider if they have problems at their end. You rserver may be slow, for example.
-also have a read of this codex article:
https://codex.www.ads-software.com/WordPress_Optimization
