Disclaimer: I am not a web developer (go figure) so I can’t provide any info based from experience.
Use the Chrome dev tools to analyse your page. Press F12 and go to Lighthouse (last tab) and generate a report : mobile/desktop.
The dev tools can also display CPU loading times, overlapping actions and bottlenecks. Also use the Performance tab and record the loading time - both full-load ‘Ctrl+Shift+R’ and refresh ‘F5’. Note ironically the refresh is more relevant as full load only happens maybe once a day - but still it’ll give clues.
Both the browser and CakePHP have significant caching, so you don’t always know what is doing what for you (static css js & images is your browser cache, repeated queries and static forms is cake).
It may simply be images which are bigger than need be, or trailing variables on GET which force ignoring the cache (which is handy at times *cough*Google KMZs*cough*).
Even the Network tab in dev tools, very handy for tracking slow AJAX, iFrame, etc.
Then, you should be able to complete the sentence; my web page is slow because of ______, how can I make that quicker? 
Edit: If its slow before loading then use the CakePHP debug and stick in debugs of function name + date_time so you can see where in the PHP its stalling - specifically focus on before & after any preloaded queries.