Citrix relies on Windows Terminal Services for doing its magic, with adding their own internal sauce to it. Imagine it's like Windows' Remote Desktop Protocol on steroids. This protocol works really quick for "Windows Primitive" elements such as buttons, captions, text, scroll bars, etc. These get transmitted really easy by Citrix to the remote user (i.e. render button in this location with this text) However, with custom elements (custom buttons, logos, pictures, etc.) it has to transmit complete bitmaps (i.e. send a bit-by-bit picture, or a compressed version of it, but it will still be an image) to the remote user, which will increase bandwidth consumption. This is a crude explanation of the remoting aspect of Citrix, but hope it clarifies some things (AFAIK) . If you're accessing outlook, internet explorer or any browser, custom mail clients with icons through Citrix, your network consumption will increase significantly.
The other side of things is how Citrix works in the background: Imagine a remote person log into a server, and then start their own applications. Another person logs in, and opens their applications, and another one. Now you have three sessions in the one Citrix server. Resource allocation is important in Citrix deployments, and typically these servers need to be beefy (i.e. RAM, CPU, HDD Space) to support many people.
I've had no problems with LTE connections with Citrix using Outlook + Internet Explorer for some clients in the past. My guess would be the network access is terrible in this case