From what I have read, 16 GB of RAM can be filled up from the SSD in just 0.25 seconds.
Thus if if a lot of programs you are running are sitting idle on and SSD swap area when you are not running it, then it wil be unnoticable to you the time it takes to bring that app back into memory.
Yes RAM is still much faster than SSD, but keep in mind you are not executing code straight from the SSD. The slower access time to SSD will only be felt if it has too be done extremely frequent such as in a for-loop in your code. Swapping a program in and out of memory e.g. because you switch between them will not be noticable at all.
Peoplel who have done videos on this have actually argued that the only reason you should get 16 GB is to reduce the wear and tear on your SSD due to swapping, not due to performance. This guys testing it cannot see a performance difference. But they can see a swap usage difference.
I dont' know if the SSDs in these new Macs are different, but if they are the same as regular SSDs then they risk degrading if you read and write from them frequently.