Is it possible to save selection in Krita for later use, like in Ps ? I cannot find that option anywhere.

There is a special layer that you can call from the drop down dialog in layers docker ( little arrow next to + icon). It is called "local selection", and it has a similar behavior to a mask, meaning that it will be attached to one layer at a time and will not be a universal selection.

-Make your selection
-Call up a "local selection" from the drop down menu. You will see a "selection" layer nested under your active layer. You can continue to manipulate the selection on that layer if you wish. You can easily activate / deactivate the selection by clicking the white circle (last icon to the right of the local sel. layer).

Keep in mind that if you merge a layer with local selection with other layers, it will be flattened. To save up your selection for later, just drag the local selection out of its nested state. This way, whenever you need to use it you just drag it inside the layer you wish to manipulate, do your thing, and then drag it out again.

An alternative to this workflow is to make a selection, fill it with a color and hide the visibility of the layer. Every time you want to call that selection, simply go to that layer and "select opaque", which will select all the pixels of that layer. The difference of this over the other method is that this selection will be universal, like a normal selection.

Hope it helps.

Thank you, This is way too bothersome, too bad there is no way to save selection without a layer and all the moving.

I have been using a layer with filled colour for selection purposes as you mentioned in you second advice. The problem is that i am working on big files and Krita becomes very sluggish as i add more layers and paint them. I really like this software, just wish it was better optimalized.

Well, the developers are doing superhuman efforts to optimize it...I guess it is a matter of time really.
I am also working with big files and understand what you mean. If you can, try saving some layers (for example a background layer with much information). Then recall them as file layers. You won't be able to work on them directly, as they are the equivalent of smart objects in photoshop. But they will make your file that much lighter.

You lost me there.

How can i save one layer and then recall it as file layer?

To export individual layers just go to Layer > Import/Export > Save Layer / Mask
You can call entire files (.kra, psd, jpg, etc) as "File layers" by clicking the little arrow next to the "+" icon on the layers docker, and selecting "file layer". This layer will not affect the performance, so it is a nice way to save resources if your workflow allows this technique. Once you're done and want to flatten the image, just convert the file layer to a paint layer, or import the original file as a layer.

That is a huge help, Thank you !