To troubleshoot please check:
The frame rate you’re getting where it plays slower and faster. You can do this in Chrome by opening the Inspector (Cmd + Shift + I), clicking the three dot menu in the upper right, selecting More tools > Rendering, and checking “FPS meter”. The expected frame rate is ~60fps. This will help you make sure there is a speed difference.
Do you notice different playback speeds on different monitors on other animations? This will allow you to determine if it's an isolated problem of a single project or something more general.
Are you using different types of cords to connect to your monitors? For example USB-C-HDMI vs USB-C - DisplayPort. You can try changing the cords and seeing if the speed changes.
The resolution. Try switching the resolution from "Default for display" to "Scaled" and select the largest scaling option.