Do you mean that only the preview is slow? If you export the video, does the exported file play quickly?
If that is the case, it might be a challenge for Shotcut to display each PNG image in real time. The work-around would be to export your PNG sequence as a transparent video.
There is a tutorial here: Tutorial: How to export a video with transparency
Then, use the transparent video in your Shotcut projects instead of the PNG sequence.