These issues may be due to memory issues, which can actually happen when there are lots of shape variants in a design, as they blow the maximum amount of kerning pairs (around 12k).
⚠️ To mitigate them, first MAKE SURE TO SPENT MOST EFFORTS ON SPACING to avoid having to create too many kerning pairs.
Unfreeze & speed up
Reload Fontself, restart Illustrator, or reboot the computer
You can use Reload Extension from the top-right tray icon or by restarting Illustrator (a computer reboot can also help if you've been working for long days):
Create fewer kerning pairs
You may also apply the Smart Kerning feature manually on fewer characters:
Use the Smart feature in the Advanced > Spacing view if you want to automatically set the side bearings values
And in the Kerning view, click Smart, but pick only a subset of characters, like just the Uppercase and Lowercase:
You can alternatively apply it to an arbitrary set of glyphs (you can view them all when loading a new font in the Live Preview, and paste them in the Kerning preview to force Smart calculations on that text selection):
Delete unnecessary kerning pairs
Remove any pair that is just a couple units (too small to be noticeable)
Remove pairs that have very low probability of being displayed:
Use the filtering field to quickly sort and delete specific characters among all the kerning pairs:
Fasten the editing
You can speed up the process by selecting and deleting all texts in the Kerning Pad and the Live Preview below: