iMAC 5k VMWARE fusion 7.01

I'm using a MACBOOK Pro and VMWARE fusion latest version. 40% of the time the VM runs ok, but frequently it just grinds to a halt and is barely useable.

I am running Yosemite. I am about to buy a new iMAC 5k and i'm concerned that if I do its going to still be temperamental and run slow

I do windows programming and need the VM for work. My other option is to buy a windows machine and run the VM using VMWARE workstation

But I really want the new MAC, but would likely break down in tears if it performed like I have now 2.9GHz intel Core i7

Please advise, anyone out there have it running trouble free?