I've been cycling on the flat surface of my SPD SL pedals. I never intend to use the clip mechanism. Should I just use flat pedals for better comfort and speed?
If you aren't using the clipless pedals with your cleats, then you have a slightly sub optimal pedalling interface. Some people say it can't be done, I do it regularly and so do you. I just do it when I'm popping out on the road bike or can't be bothered to take two pairs of shoes to work.
If you just never clip in then I would get rid of the clipless pedals and use something more suitable. You could try flat pedals, but there are also converters. This could be the cheapest and would be totally reversible, you could pop them out and still go for a clipped in ride with no hassle any day of the week. Try searching online for platform adapters, pedal adapters, pedal plates, clipless converters etc.
The velodrome near me uses something similar to convert their hire bikes from clipless to toe cages, for new riders who want to try the track but without the expense of dedicated shoes.
A flat platform will improve your comfort in thin soled shoes, and it will improve the grip you have on the pedal, increasing confidence and maybe safety around traffic in particular.
So yes, make a change, but it doesn't have to be new pedals necessarily.
To my knowledge there are no inherent benefits of using different flat pedals in terms of performance. Larger and more massive flat pedals tend to be heavier, but unless you are a professional cyclist the extra ~100 g should make no difference. This is, of course, assuming that both your SPD and flat pedals are properly maintained and the bearings rotate freely. You are likely to be more comfortable using flat shoes with flat pedals, or clipless shoes with cleats with clipless pedals. You are free to use whichever pedals you want (or have available) with whichever shoes.
However anecdotal, many cyclists will agree that they find using clipless pedals is beneficial as far as stroke smoothness or power delivery go. If it's an opinion you're after, I bought my carbon road frame for speed and distance, and it uses clipless pedals exclusively, even if I have to hop into town with it with sneakers on. My steel frame has flats for the city, and I swap the pedals out for clipless if I take it on a longer ride.
As you're likely aware, it is difficult to walk around in cycling shoes with road cleats (Look Keo in my case). Note here that some shoes and pedals, primarily marketed to MTB cyclists, (such as the PD-M520) are made so that the cleat is recessed (i.e. "hidden" from the road surface), which enables normal walking.
There are options available to cyclists who use their bicycles both with flat shoes as well as cycling shoes with cleats. Some of these include (1) adapters for clipless pedals to in a sense "convert" them into flat pedals, such as these Look Keo adapters or (2) pedals engineered from factory to have one side flat, and the other side compatible with MTB SPD cleats, such as these Shimano PD-EH500 pedals.
If you are not going to use clipless shoes the advantage of flat pedals is whenever you put your foot down you have the right side of the pedals. With the pedals you have it is possible to put your foot on the clipless side. I don't know if you find that a problem, but that is what you will solve.
When I bought my first modern bike I bought pedals like you have and bought clipless shoes. I wasn't willing to commit to always wearing my biking shoes when I rode the bike. I am primarily a recreational road cyclist, riding 2+ hours at a time, so I am willing to change clothes. I imagined there would be times I rode, probably for transport, where I would want to ride in other shoes. After five years I was buying another bike. In that time I had ridden a few times for transport but had chosen to put on my biking shoes each time. Sometimes I was going shopping and we were short a car. I just walked around the store in my biking shoes. Sometimes I was going to play racquetball and I carried shoes for that. I learned to get the right side of the pedals most of the time, but not always. When I bought the new bike I put on clipless both sides.
You don't say how you ride or why you have the pedals you do. If I were riding primarily transport I would want to ride in whatever clothes I wanted at the other end, would not be riding so far or so fast, and would have flat pedals. The pedals you have are for someone who want to ride each way at different times. If that is not you, the pedals are not right for you.