When I had sub 30 frames I couldn't look around or aim confidently. You lose initiative simply because you are reacting to the delayed actions and compensating by waiting to see what you feel like you can do, not just doing it.
Even a constant 30 fps allows you to be more confident, peek and shoot, etc.
Certain AI with the shotguns have a zombie mentality and by running away you can sort of corral them into choke points, like up stairs and such.
Keep your distance, aim down the sights and let them line up to you. Importance of having higher frame rates, the moment you take down their shield, swap to your kinetic weapon and try to take them down, you have a 2 second window where they stagger so the quicker you see and react the more damage you can inflict.
Also why frames are important, the semi automatic pistol has a maximum fire rate, and clicking as fast as possible actually "misses" the mark on achieving fast shooting. The more smooth your rendering the easier it is to process controlled clicking and also recoil.
If all else fails, an upgraded rocket launcher found at vendors is great at finishing off enemies, due to the stagger animation when shield drops, and followup stagger on the second rocket, gives you time to swap weapons of they're still not down.
You can also call an Apex taxi and sit on top of ship, use its shield to protect against long range weapons and take out the ones that come closer within the ship shield bubble.
Playing in low conflict zones helps you acclimate to the fps aspect, movement and such.