First of all, if you do plan to do much in the video and high-resolution photo area, you will find 1 TB fills up pretty fast! But to your main question, a single large drive is NOT generally slower than a single smaller drive when the each have similar amounts of data on them. The system does NOT "wade through all that" to access stuff. A HDD is a "random access" device, meaning that there are efficient ways to find anything placed anywhere on it. The system essentially involves a series of indexing files (the directories and sector allocation systems) that tell it exactly where on the disk to find the particular file you want. "Where on the disk" is defined (internally within the HDD unit itself) in terms of address numbers for which disk in a stack of disks, which head unit out of two (top and bottom surfaces) per disk, which track on the disk (from inner to outer edges) and which sector within that track.
Probably the most important factor in the performance speed of a HDD that has been used for a long time and has many files on it, with many of them having been revised multiple times, is something called Fragmentation. As any file is created it is assigned to start at a particular sector and keep on adding sectors as required to store all the file. Ideally, those sectors added should simply be the next one on the disk which is quick to find and use. But as files are expanded by adding more sectors, often the "next sector" is already occupied, and so a different one has to be found somewhere else. As files are deleted, their sectors re-enter the pool of available ones, so new sector assignments can be made from almost anywhere on the disk. After a while, we find that a large number of files are stored in many different blocks of sectors each, and the disk heads have to do a lot of re-positioning (much slower than just reading off data) to get to them. This i called Fragmentation - many files are in separate fragments rather than all in one continuous string of sectors.
All systems, including Windows, have utility tools called Defragmenters that can fix this problem. They take a little bit of time to do the job, but when used periodically they can prevent Fragmentation from getting so bad that your disk actually does seem to be slow. Basically, what you do from time to time is two steps. First you Empty your Recycle Bin. This is the place where old files you said you wanted to "delete" really are saved on your hard drive in case you change your mind. So before emptying it, it is a good idea to review what it has in case you really do want something it holds recovered (and easy task). But assuming you don't need anything in the Recycle Bin, you empty it and the files really ARE deleted and their disk space really is freed up for re-use. Then you run the Defragmentation tool. What it does is study all the files and their locations, then move bunches of them to a temporary location in unused disk areas, freeing up large blocks of space near the beginning of the disk. Then it copies all those files back to the newly-freed large block of space, making them all use continuous blocks of sectors so there is much less sector-seeking to be done later. It continues this process until all the disk files are in continuous blocks. When this is done regularly, even an old well-filled large disk can perform pretty fast and is VERY close to the speed of a smaller disk.
RAID0 is a different system, and it has some characteristics that can speed up certain types of disk operations, depending in part on file sizes. Your planned use includes both many small files (gaming) and very long files (video and photo), so the potential advantages of RAID0 will apply only to part of what you do. Moreover, some of those advantages are very small given the very fast hard drive units now on the market. Gamers often claim RAID0 is an advantage, and that certainly was true a decade ago. Whether it really is true now, I can't say - I'm not a gamer. I suggest you read actual reviews and lab tests published on websites like Tom's to see how much difference a RAID0 makes over a single modern disk doing the types of work you plan.