What can I move from OS (C:) to DATA (D:) on Inspiron 3780 to free up space and speed?

I'm a UK-based Inspiron 3780 user. It has a OS (C:) drive and a DATA (D:) drive.

OS drive fluctuates between 1GB and 9GB free of 103GB; laptop runs more slowly the lower the free space; free space reduces with use, increases by defragmenting/optimising.

My Data drive has 194GB free of 931GB.

Is there anything, or any category of thing, I can safely move from the OS drive to the Data drive which will free up space and increase computer functioning speed without breaking the computer? I don't really understand what things need to be on the OS drive and what things don't.

Any help/wisdom much appreciated.