The white spots at the backside of leaves could be aphids. Dish soap water is good for them. But the bigger holes on the leaves are some other kinds of pest. Could be snails of grasshoppers.

The spinach will grow faster as the days get longer -- but you have to clip it, too, to encourage it to grow. Clip the leaves that are quarter to silver dollar size and use them now.

The insect damage doesn't look bad at all to me. THink of it as sharing.

I've had spinach stay at a stand still in store bought soil in the one raised bed I have. I guess it could be a soil issue, if those plants are really tiny (can't tell the scale in your photo.) Someone else may have a suggestion for that.

You should have thrown in a lot more seeds in ther than one could not see the bare soil.

As far as the growth, They will probably pick up the speed very soon when it warms up a bit.