Compression has no impact on performance, only on the size of APK.
Image size has obviously a huge impact on performance and memory usage. The bigger is the source image the more memory it takes no matter what sprite object size you have in your project.
Also engine has to up/downscale the image for the sprite object which takes CPU.
But a flappy bird like game has no right to be laggy. There is something wrong in your project probably. If you can send me the capx, then I could try to optimize it.