Why Do Large Documents = Horrible Performance?

Could one of you wonderful Germans familiar with Open-Xchange's documentation explain to me why large mailbox Drive documents perform so poorly and how this problem could be fixed? Some of my documents take forever to edit, the cursor just drags, everything is painfully slow, copy and paste becomes a perilous expedition. These are .docx documents created within Mailbox's own Drive interface. It's so bad that I need to create multiple documents to cover one subject because Drive simply can't handle all that text. It's just text. This is 2025. How is this still an issue? It has nothing to do with my internet speed or fancy graphics or running out of storage space. Small documents with only a few pages of text work fine. This feels like bad programming, but I'm asking for confirmation.