Is TrueNAS/ZFS a Good Solution for 99% Disk Usage for Streaming?

[HarryMuscle]

I keep reading than ZFS requires free space to have good performance, however, I plan to use it to store large files for streaming and would like to fill it up almost completely. If my pool is 10TB in size and I were to fill it to 99% capacity leaving 10GB of space would ZFS still function? When people refer to poor performance in such a situation what are the actually referring to? Will it drop to kb/s in read and write speed, or is it more like 90% of the original performance? Any other issues to watch out for in such a use case?