Hi 
42Mb is extremely large for a spreadsheet.
1. Perhaps try using Gnumeric instead? It's a dedicated spreadsheet
program so it is streamlined specifically for dealing with
spreadsheets and doesn't have to worry about any of the other
apps/modules taking up resources
https://projects.gnome.org/gnumeric/
2. Do you have a lot of embedded images or anything like that?
Perhaps link to them instead of embedding them? If you do then it
might be best to put the file in a folder of it's own and then put a
short-cut to it where-ever you currently have the file. Then all the
images can be in the folder with it, or copies of them, so as to make
it easier to keep track of the images.
3. Are you using the spreadsheet as a database? It can be a good way
to start off but you have long past the point where you need a proper
database. You will find the total file-size probably drops
dramatically when you do move it. To start with you might find it
easiest to use Base to read the existing spreadsheet but it would be
better to start moving all that data into a more suitable back-end.
4. In LibreOffice have you changed the "memory" settings yet? If not then
Tools - Options - Memory
and dramatically increase pretty much everything in there. I usually
go as high as 20Mb for most things but occasionally go much higher.
You might need more like 60Mb. Perhaps reduce the number of steps you
can undo. 100 is possibly excessive so perhaps drop that to 50?
Just to help other people give better answers please let us know what
Operating System you are using. Is it one of the Windows ones? Also
it might be interesting to know how fast your cpu is and how much Ram
you have although these last 2 facts probably wont help us help you.
Regards from
Tom 