My game was functioning at 60fps consistently until I added a quad background.
The background is a quad with the Unlit/Transparent placed behind all the scene game objects. It is very large so that it covers the whole screen when viewed by the camera. It is not static, it moves exactly as the camera moves to keep the background fixed. It has shadows disabled (both casting and receiving). It uses metallic property of the shader. It has light probes and reflection probes disabled. The lag happens periodically when the FPS momentarily falls to about 40 fps.
Thank you for your help
Are you still clearing the background with a color?