I absolutely agree with Kyle and Karthik here - MSTP is internally built on RSTP and if properly configured, it has the same speed of convergence. If you are having issues with MSTP convergence times then it is not configured properly, and you are not going to save yourself any work by performing a network-wide downgrade to RSTP. You could even run into issues if the number of VLANs created in your network exceeds the maximum count of allowed STP instances on your switches.
It would be better to inspect your MSTP configuration. How large is your network, how many switches are there and what is their MST configuration? How do you measure the convergence times and how exactly do you know there is a problem with it?
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...