The difference between the two modules may be explained by the fact that Develop module builds the image from the original data plus the saved edits whereas Library module displays a prebuilt preview.
Obviously as you have noticed a change something has caused it, there can be a number of reasons:
Try Optimise Catalogue
How much RAM and are Modules all performing properly
Does turning off GPU acceleration change anything?
Preferences>Performance and uncheck your GPU options do things improve
Investigate any updates to application and drivers prior to noticing slow down
Check HDD free volume you need at least 25% free for efficient use
I assume you do this already but your Lightroom Catalog (and preview files) should be on your computer main drive not external
Final thought to fix buggy performance reset preferences may help
Hold down Alt Shift (Opt Shift MAC?) and answer yes to dialogue box. This will set ALL LR prefs to defaults so you will have to reset those that you need back to your needs