Regardless of the capacity, whether 8 GB or 1 TB, the system slows when you have little or no free space. Cloud services in particular require a lot of cache/buffer. So if you have a 1 TB iPad... try to keep 3-5 GB Available. Otherwise? Why would having a lot of data sitting in storage have a negative impact on the rest of the system's performance? Mobile devices are no different than desktop computers in that regard.
As far as managing that storage, it'll undoubtedly depend on what it is you're storing. To my mind, the primary usage for 1 TB in an iPhone/iPad would be shooting 4K video. If you're shooting professionally it's likely you're going to be offloading that video on a regular basis, and it's likely the size of each file will be a more serious consideration than the number of files - bigger files, longer transfer times.
People have tended to treat computer storage as if they were snails carrying their entire home around on their backs. All too often housecleaning is indefinitely delayed in favor of simply increasing the internal storage capacity from one computer to the next. That kind of "management" is not management at all. While a desktop OS does give you more tools for managing massive numbers of files, how many of us really would need to manage massive numbers of files if we didn't continually kick the housekeeping can down the road?
Cloud storage can be a big help in this regard. Most cloud storage systems can be designed to automatically offload files that haven't been accessed in a while (an approach used by iCloud Drive and iCloud Photos, among others). Since the "real" data is up in the cloud, it can be managed from a desktop PC using (potentially) more robust file management tools.
That same capability of cloud storage systems is also a help in reducing the need for on-device storage (whether mobile or desktop). If the same data needs to be accessible on multiple devices it eliminates much of the redundant storage of that data, downloading the files only if actually needed, automatically removing them if they go un-used for a period of time. My 64 GB iPhone has 19 GB free space, my 128 GB iPad has 78 GB free space. I have 150 GB in iCloud.
Now, I don't shoot Prores video (professionally or as an amateur) - I'm mostly a still photography guy, so others would have to weigh in on their experiences with massive data files.
