You need the proper density for good to above average roaming.
The negotiated speed represents the data rate that the client settles on based on signal strength and quality. If there is congestion or interference, it will drop. If the signal strength is too low, or the client is too far away and it cannot send traffic successfully at the highest rates, it will negotiate down. It takes longer to negotiate up on the same access point.
With that being said, good coverage is necessary for any client to maintain good datarates, at a minimum.. One access point every 60 feet in an office environment with drywall is a good starting point.