I'm using latest beta version of osu but although I can't confirm right now, I remember the same glitch happening before installing the beta too, whenever I'm playing a map (in taiko at least) with music that starts right before the first note (e.g. pulsefire trailer), there is a short but visible fps drop (a few dozens of frames telling by eye), it could be related to the audio buffer. The fps drop is not visible when the music starts before the first note appears on screen, or at least it's harder to notice.