There are still application vendors today that will not commit to saying that their application will work properly under Citrix, or VMware. In some cases, they even know that there are active problems. ERP apps can be tough nuts to crack, because in a lot of cases, you're getting all the install and support through a VAR, so you might not have access to a vendor KB or support forums.
It's possible that there are known problems though - if their application does stupid things like a ton of small disk writes, it can cause performance problems on a multi-concurrent-desktop-user system like Citrix or TS, especially if you don't have things like a write-cache on your storage system. Some software, and I've seen ERP software like this, assumes that only one user will have all the system resources available as needed.
So, push them for evidence of the problem - under what conditions is performance impacted, do they have sizing guidelines, etc. It's possible that your install won't fall prey to what he's claiming is a known problem.
However, if it's a real problem, and your testbed isn't generating the production load (real number of concurrent users pulling order reports or doing end-of-month activities, real stress-tests), then you can't do a good job of proving him wrong, either.
Would the speed of the virtualized app not just depend on the resources/network connection the citrix server has?
In general yes, but as I said above, it's not just CPU, RAM, and LAN bandwidth. There's also things like disk IO, and weird Windows things like Session Desktop Heap and GDI counters, that a single-user system very rarely has problems with.
Best Answer
Simple answer is "Yes." The ICA protocol was developed when dial-up was a very common internet connection : if you tuned it properly, you could fit a single ICA connection into about 8-10 kbps. You can add, remove, and rate-limit all the channels for things like file transfers, sound mapping, removable devices, etc.
RDP is a lot like ICA but not quite as configurable.
VNC is a remote framebuffer - in its original iteration, it was a think-tank project for an ATM LAN, when 155 Mbps was insanely good. Nowadays, many flavors of VNC have lots of different compression options, so it's better than it was.