NASA uses a lot of spade connectors, the only thing wrong with them is that they're somewhat bulky, which is common in legacy connectors. You shouldn't experience any warming with properly installed spade connectors, properly rated. Something else must be going on.
First, soldering is NOT better than crimping. Again, NASA (and the DoD) don't ALLOW soldering of crimped connections, because crimping is better. A proper crimp connection exerts enough pressure to cold-weld the copper You just need to be sure the crimps are done correctly. Use a good crimping tool (which can be expensive), or learn to properly use a cheap tool (they can make good crimps too, but you need to use them properly -- the expensive ones are pretty much foolproof).
Are you sure the heat is coming from the spade connectors themselves? I'd be far more suspicious of the cable-end connector. Those experience a lot of connect/disconnect cycles and the contact fingers can loosen, causing high resistance. The second thing I'd look for is oxidation on the pins or sockets of the connector. You can clean them by gently scraping them or just replacing them.