I wonder if you had some user profile corruption in your LO7 installation that was the cause of the slow down? I use the manual method of renaming the user folder so I can then copy back the template, wordbook, AutoCorrect, etc folders.
The memory setting is probably not the cause: These settings don't control other memory-related aspects of LO; and they generally are unrelated to performance issues in LO.
The default user memory limits have been increased with each new version. However, if you had changed the default values in LO 5 or earlier to a user defined one, then that old user-defined limit stays during updates (tdf#110448) and might be well less than the default in a brand new installation. I suspect that resetting your profile might remove any user-defined memory limit.
BTW I, like @Hrbrgr, assumed the image sizes were in kb as that was the only unit shown.