this is time to keep eye and pluck the issue,simply use wp fastest cache and rebuild cache ,if response is same then do some
turn off all the plugins and check the response if its been ok then means theme is fine if not then check your theme header file may be lot of out resource jquery or some are used.
seems its ok after turning off plugins then try to turn 1 by 1 plugin and check response
you will find a plugin which may cause for high load
further more try to use 3rd party websites for to check website speed like speed test instead of your local browsing
WP is basically resource suc***r so have to keep strong eye on resource panel