So this just happened, one of our vmadmins came and complained that the console used 5 minutes or more to start - just sitting "idly" on the splash screen. In disbelief I started my own console, but unfortunately he was right. This is something new, as it worked just fine yesterday... it's not using CPU or RAM, the VMM server seems okay (no load to speak of)...
We're running SCVMM 2012 R2 with the update roll-up 5 installed ... (both on clients and server).
To the best of my knowledge no configuration change has happened the last week with regards to the VMM server or client computers...