Can you consistently reproduce this abnormal behaviour? Does it apply to everything that you do which requires a connection to online services of Mendix? (home.mendix.com, connections from the modeler to the teamserver etc...)? I suggest to create a support ticket for this, so we can work toghether with you on this issue.
Did you already eliminate the possibility of this being a problem of the location where you are working? (transparent proxy servers, whoknowswhat?) How does using home.mendix.com and everything perform when you are working at another location? If you create a new project / sandbox, is the responsiveness of the portal as bad as in this one project you're talking about? This can help debugging if this is an issue related to a network connection, or a specific project, or even other things. I can assure you that this is not a general problem that's occurring for lots of other users and a lot of projects.
I've seen the same issue in a customer project, more or less reproducible, and already filed a ticket for it. Please refer tot ticket 203407 and ticket 102080
Regarding the DevPortal part:
We encountered some performance issues in the Development Portal which caused it to at times become slow and sluggish from Thursday to Monday. We managed to find the root cause in the meantime, so DevPortal performance should be back to normal now.