Hello,
Sadly you did not share your website so we can't really speed check it right now and figure out what's causing its low performance. That being said, we can do it all, why not.
Lets NOT ignore the code, lets go through the Client-side and make sure that everything is optimized AND minified, so javascript and css files don't have redundant or useless codes such as tens of loaded and unused fonts, which seems to be very common, or functions that take forever to finish. This is very important because lots of devs just use random libraries and frameworks along with copy pasted code everywhere.
We can then move to the back-end where what matters most is how long are those SQL Queries taking to return results and how are they being executed. If there's something we don't like we can either edit the tables by setting up Indexes, which will fasten searches, or simply make sure that our PHP code isnt taking forever to parse the results. Not everyone is a DBA, some devs don't know what Indexes even are and that's just a mess.
Let me take a look at your website, tell you what I think, point out a few things that might be causing this, we can talk about it and maybe then you can chose me. I'm good at what I do, let me handle this one for you once and for all.
Thanks for reading.