on opengl (webgl more specifically) for textures, which format between jpeg and png would be recommended as more efficient at runtime rendering? I understand jpeg can be smaller than PNG, but perhaps decoding can be more costly, or otherwise.
Well decoding a JPEG happens really fast, you would need to use thousands (or hundreds maybe if you are on mobile) of high resolution JPEGs to see a substantial hit on the performance. As your plattform is web I would recommend JPEG, depending on what kind of textures you have it would be the most size/quality efficent format that is supported on all browsers (webp being a good but not well supported alternative).
If your textures are more graphical or cartoon like, meaning they have a lot of sharp and fine edges and a lot of single color areas I would recommend PNG, hard and fine edges can get very smudgy with medium-high JPEG compression.
PNG does lossless compression. Specially with cartoon like content (meaning very few colors and details compared to a photo) you will get a similar size of a well compressed JPEG but with better visual results and in some cases even better compression.