Efficiency gains once your foot moves as one with the pedal are very, very small. The gains from the traditional clotted aluminum cleats, toestraps and clips to anything now is basically zero
UNLESS your feet, knees and body need float to do their best or not suffer injury. (Or you are one who will suffer crashes and injuries from the mechanics of toestraps.)
SPDs give you that solid connection. All other systems can do is offer different amounts and types of float which may or may not matter to your feet, knees and body, give you different releases, different weights, pedal clearances, sex appeal and cost. (And walkability a+nd convenience).
For me, float is not good. I can ride SPDs if I crank the toe-in a lot on the left cleat and to full max on the right. Black no-float cleats and LOOK Delta copy pedals work very nicely for me. So do the old clips, cleats and straps. Once I am riding the pedals don't matter until it is time to stop. I can do the same 60 mile loop on my fix gear with toeclips and my good bike with LOOK cleats and have the same comfort, efficiency and power (except one is fix gear and the other geared; now that is a huge difference! I could swap the pedal and that wouldn't change the ride. I might however crash - from not remembering on my good bike or pulling out on the fix gear.
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Look at whether the SPDs are serving you well for foot, knee and overall comfort. Also how these shoes fit into your life and riding style. Think that last part through for any changes you are considering, If you change, keep your old shoes and cleats set up. Keep the pedals. You may even consider swapping pedals for different rides. SPDs most of the time, the new ones of weekend long rides and events. (Swapping pedals is easy and fast
IF you don't honk on the wrench when you put them on, In fact, pedal threads self-tighten. A 6" hex wrench and ordinary hands will get them plenty secure. Pedal wrenches are huge because mechanics see everyday bikes with pedals honked on decades ago.)
The old custom bike of my username now runs fix gear with traditional toeclips but I've been known to swap out pedals and shoes to SPD to ride it in town when I am spending that day indoors. Toughest part of the swap - remembering so I put on the correct shoes!
Edit: And yes, there is a learning curve to getting your foot into any pedal systems and some are easier than others. It often take me half a ride to re-calibrate after I switch pedals or bikes. Second edit: For many of the LOOK cleats and similar form other manufacturers, KoolKovers makes rubber covers you slip over the cleat when not riding. Huge benefit! The cleats last much longer and yo are much less likely to break your tailbone sitting down hard on a waxed supermarket floor. Going down stairs is much less scary. They come with a clip; basically a huge paperclip, but the little key ring carabiners from REI work much better. I hand them from the back of my toolkit. Yes, it looks dorky. I put them in my jersey pockets for years, resisting the dork bit, Finally switched, So much easier! Less hassle. No dirty pockets or plastic bags to mess with. More room for other stuff (and better on-ride access). So I look like a dork. A dork with a tailbone. Oh well.
Ben