Cadence Virtuoso 6.1.6 is EXTREMELY slow while simulating
You are right. The problem is the voltage source you have in the feedback loop.

You're trying to build a VCO, the frequency of which i believe you're controlling with the current level of the current starved inverters you have in the middle cell. Your frequency of oscillation will be determined by the propagation delay of the inverters in your chain, and trying to force it to another value with a pulse generator in the feedback loop will simply make the simulator go bonkers, as you're trying to drive a fixed frequency to a positive feedback loop that is trying to oscillate itself at a frequency determined by circuit components.

Remove the pulse generator, run a transient simulation to determine your oscillation frequency. You can guesstimate it by adding the propagation delays of the inverters you have in the chain.