SQL Server AG - Is there any case for un-clustering prior to certain operations? I have a software company asking me to remove a database from a synchronous availability group prior to them running their upgrade scripts.

They are convinced that this will increase the execution time. Are their any DDL or DML queries that would take significantly longer when clustered?

What I want to know is: If I do an operation that takes 5 minutes on a non clustered database, will the same take 10 minutes in a cluster, or the same amount + some minimal overhead? I'm guessing it's not going to double or even get that close, but I was hoping for some experienced insight.
Yes they are correct it will increase the execution time. As the comments state though there is no measurable way to determine this unless you have a matched test/stage environment and try it both ways (which would also be a best guess since you cannot know latency and bandwidth of your network and the load on the nodes). 

I see this as the company asking you to trade HA and your time (adding the DB back to the AG) to save some indeterminate, likely small, overhead for their scripts to run faster. They better have a pretty good reason to trade that.