A nasty problem for keyboard navigation (especially in Caja) came up recently in 18.04 Daily and I'm glad I did some new-feature reading and was able to locate why.
In Caja, the quick keyboard navigation makes use of ALT, mostly combined with Arrow keys. But it was a frustrating fight to get it to work and took several hits before it did anything. I could see slowing down helped.
Enter HUD which apparently uses the ALT key and a 250msec delay. BINGO! Disable HUD in Startup and Caja is its old friendly self again. It's just a battle for the ALT key.
Hope this helps someone else.