Is this 1 TB internal drive a hard drive (HDD) or a Solid State Drive (SSD)? The latter is much, much faster and if you're going to be tinkering around with the innards of the machine anyway, might as well go with an SSD. They come in 1 TB and 2 TB capacities, and I think larger as well for internal SSDs. As suggested, if you've got space inside the machine you could simply ADD a second drive rather than replacing the one that is in there now. There should be space, as SSDs are significantly smaller than platter HDDs.
In answer to the question about whether or not external drives are slower than internal drives.... SSDs are fast, much faster than any internal HDD. The USB-driven ones such as Samsung's T5 are nearly as fast as my internal SSD, and Samsung's X5 NVME Thunderbolt 3 SSD is as fast, or darned close to it. External HDDs tend to be slow, yes, usually only 5400 rpm, although occasionally one will find a drive at 7200 rpm. Now manufacturers seem to be using a new strategy which results in even the HDDs are faster than they were in the past.
Back in 2015 when I bought a 15" MacBook Pro, which is not user-configurable/updateable, I was moving to it from an iMac with a 1 TB HDD, while the MBP has 512 GB SSD. I had to do some major paring-down, needless to say, when setting up the new machine! I realized at that time that I was going to need to provide room for growth, too, although I already was using external HDD for standard backups. It was right about then that Samsung came out with their T1, an external SSD which was the answer for me. Since then I have continued to use external SSDs to supplement what is on the internal SSD, even though I now do have a new machine with 1 TB SSD. This system has worked out well for me. Anything that I don't need to keep on the machine -- that is, any photo files that have been completed and are not in some stage of the culling or editing process are shifted over to a T5 and it only takes a minute to plug in the device when I need to retrieve or access a particular image. I also do the same with documents, videos and individual music files that are not in iTunes. This leaves plenty of breathing space on my computers and yet anything I need is still quickly accessible. Anything which I know I'm really truly finished with the eventually winds up in the older HDD archival drives. Everything is backed up regularly, too.
Just some food for thought.....