ok,
waterfall view says you have issues lol, 23 seconds for the FIRST BYTE.
Then your loking good on all element coming over at normal speeds, about 4 seconds for the entire page to be processed.
Here is what you need to check.
All programming codes, loops, SQL query
DId you make any changes to the site? Plugin, tracker or anything simple?
If you want I can look closer if we are on at the same time, but as I stated above you need to make a tst page or two.