50 or so tasks shouldn't be slowing down OmniPlan. What's the project start and end date? If there's a task end date set out way into the future or a project start date set way back in the past that could be causing performance issues if the gantt chart is trying to draw all that time in between.
If you create a brand new OmniPlan file with the same amount of tasks does it still lag on your Mac?
Could you send us the MS Project file so we can try it on our end? Our support email address is
omniplan@omnigroup.com.