I've
attached a project that has cursed automation item. It only has a few visible nodes, but appears to be 4MB of data in the project file.
No clue why Reaper created it. This was part of a bigger project. I isolated the hanging issue to this item by trial and error.
With it there, adjusting the envelope fader hangs Reaper. It becomes totally unresponsive for several minutes (on my machine), then comes back. Trying to adjust fader will hang Reaper again.
Gluing the item doesn't fix it. If you save the automation item, it's 4MB. Trying to reduce points on it hangs Reaper. If you drag out the edges, the are fewer than 10 visible points in the loop region. If you glue it like that, it just gets even more giant internally.
Windows 10 (19041) and Reaper 6.29 (latest as of this posting).