Could you provide some additional background information on your case?
* How big is the project on disk? Is it a new or existing large project?
* Do colleagues experience the same issue on the same project?
* Did you experience the same lag in previous modeler versions? Or was it introduced in Mx 7.8.0
* Is the model stored on a network location or OneDrive, Dropbox sync folder?
* Is Anti-virus scan running?
* What's you system config? Operating system + hardware config
* Could you test whether you experience the same issue on a newly created project?
Maybe this question gives you some pointers: https://forum.mendix.com/link/questions/15822
Are you running any other programs at the same time? I would try only running the modeler to see if the issue persists.
If that doesn't solve the issue, while running the modeler, check the performance tab in task manager to see how much memory and CPU utilization is being used to run the modeler. If they both are pretty high, double check that your computer meets the minimum requirements to run the modeler.
Here is a link to system requirements but they don't really say anything about minimum computer specs:
See also https://forum.mendix.com/link/questions/88806
We see the same problem in Mendix Modeler 7.17.2. I just found out that the trouble starts after opening an extremely complex page in the modeler. If you then close the modeler and then rename the project folder, it will be fast again.