I always have issues with systems with only 1 GB of RAM.
Do you have a lot of disk activity when you open the file? You say 5.5 GB free space. That seems low somehow. Have you installed "disks" or "gparted"? I think you might want to look into the size of your Swap partition. I have 4.1 GB Swap partition on this laptop. That was what Ubuntu created automatically during the install.
I know that Linux ext4 do not need defraging like you need with FAT/FAT32/NTFS Windows partitions, but the Windows rule of not going below a free space of less than 10% of the partition's size. On low RAM [ 2 or less GB ] systems with 80 GB, 250 GB. or 500 GB drives, that I have created in the past [ both Ubuntu and Xubuntu ], seems to take longer when I get below 10 or 20 GB free space. It is even worse when you combined with a slower single/dual core processor.
I gets worse when you figure in the fact that a lot of built in video cards shared the RAM with all the system's RAM needs for running the OS and the packages. The lower available RAM, the more you need a good SWAP partition size. I have a lot of experience with taking 32-bit and 64-bit processor XP systems to Linux boxes. Even running Xubuntu instead of Ubuntu did not help much getting good performance with those systems.
Yes, having Calc take so long to open the file, with no problem moving desktop windows around the screen, seems "weird". But, low RAM issues can create "weird" issues, in my experiences over the past 10+ years using Linux on them.