Hi SReitsma and welcome to our forum!
Are you saying that InfoPath opens slowly? Can you quantify that? Is it 1 second or 2 seconds or 20 seconds? SharePoint lists are HTML pages rendered on the server and since you already have Internet Explorer open the display time is quick. When you click on an InfoPath icon, it has to launch InfoPath. Try leaving InfoPath open and clicking on a second document. Is it still the same speed? How big is the InfoPath template? The first time you click on an InfoPath form, InfoPath will cache the template on your local machine and if your template XSN file is large, it will take time to download. How big is your XML file (the form data)? If it contains a big image, it will take time to download to the client. InfoPath is running on your local machine, so it could also be a hardware issue. Do you have enough memory and is your processor 2.5GHz or above?
What we have found is that opening an InfoPath form from SharePoint is slower than from a Web service. For modestly sized forms, SharePoint opens can take between 5-10 seconds depending on various factors. Web service opens tend to happen faster (<5 seconds), but these numbers are highly dependent on server, client, and file sizes.
Patrick Halstead
Project Manager at Qdabra