This can be espected as squid "engine" is not that good for dynamic/session pages.
I saw it on one doc comparing squid and varnish.https://www.varnish-cache.org/trac/wiki/ArchitectNotes
A test I did once was configuring a varnish daemon as proxy server sending to a squid as backend. The result was some times faster then squid itself.
But I'm sure that there is some config options to improve cache hit.