It seems that Roblox has a memory leak sourced from either core scripts, or in-engine itself. With my own tools it is impossible to determine. In the below attachment, my memory went from ~400ms to ~570ms in just a few minutes.
Repro (all default scripts are disabled):
MemoryLeakRepro.rbxl (203,9 KB)
If anybody has any further information, feel free to let me know.