I'm interested in speedruns, and have wanted to try my hand at it. I just wanted to check if the internal SSD swap would immediately make all runs illegitimate. I have no problem sticking games on an external usb 3.0 ssd if that's the limit.

All games have their own rules for what is legal and what isn't. You'll need to check the rules for any game(s) you want to run and/or you'll need to contacts the mods/community for the game if you have further questions or if the rules don't clarify your question.

Thank you. I wasn't sure if a storage mod like this might have a blanket ban or not.

In the case of PS4 it's usually allowed because Sony actively supports and encourages you to purchase and insert compatible drives into the console. Now I don't know if something like that is supported natively on an XBox One, but most (not all) communities only ban modifications to consoles if they are not supported by the company who made the console (for example, you can use the PS2 HDD to install files from games that support it like Final Fantasy X International or Kingdom Hearts Final Mix, but you can't use homebrew applications to load actually full game ISOs from an attached HDD because that was never supported by Sony). But again, these are just examples and ultimately mods/games/communities have the right to ban, or allow, whatever they want to.

Thank you. Looks like I'll be best playing it safe using external only since a hard drive swap is not officially supported by microsoft