Storing to a database should provide a noticeable improvement to file size (perhaps reduce it from 75 MB to about 40 MB or less) and therefore noticeably improve the speed of loading/saving - and the save speed will be improved even more if you keep DataTable(s) open while your program is running (as then only data that actually changed in some way needs to be saved). Depending on how much data tends to change each time, the save speed could be improved very dramatically.

Unfortunately setting up the database and code to save and load the data will take quite a bit of effort, so it isn't a 5 minute fix.