Adding some high mass/drag payloads to aircraft can impact their performance significantly. You will now have a visual guide when your aircraft are overloaded in the loadout screen

When selecting new weapons, when an aircraft is near it’s max capacity some payload options will be shown in orange. You will still be able to select them but will have reduced speed and maneuverability until you expend them.

For some units these warnings are hard limits. A walrus can mount up to 2 AA missiles, but only if it has enough buoyancy spare.