Landing pages typically take the longest to load the utag.js and vendor libraries because they have not been cached yet. They will load very quickly regardless, but once the files have been cached they load even quicker.
What could be happening is on the initial page load of a landing page there is a longer period before the DOM Ready event happens, which stretches out the time before Tealium will trigger the tracking pixels, like GA. This leaves the door open for users to navigate away from that initial landing page before any tracking is done. Without tracking on that first page GA has no way of knowing where that user came from.
Here are some tips to increase the chances your analytics tags will fire soon as possible:
(1) Update to latest utag.js template (version 4.2x or later)
(2) Use Bundling feature - Loads the tags in utag.js so they load quicker
(3) Set "Wait = No" for your Analytics tag in Advanced Settings
(4) Tag your utag.js on your page at the top of the body (as recommended in the Tealium Code Center)