The performance of these saws is very dependent on the appropriateness and sharpness of the blade.
Some of the jig saw blades of my assortment can sharpened by hand, using the same technique as used for sharpening handsaws.
However, some blades now have the same treatment as "hardpoint" saws, making the teeth very hard while the body of the blade is softer. Firstly check with an old triangular file.
You will need a 6" or 7" triangular saw file and handle.
Also, either a metalwork vice or improvised chops to hold the short blade, and a candle. Use the lighted candle to blacken the teeth so that you can see where you have worked.