Two of the most frequent issues with performance is data loading and waiting for a table to display.
Data load performance can be significantly improved if the Preference (F10) "Use in-memory journaling" is set to True. The database can become corrupted if the app is shut down in the middle of a database update by power failure etc. As a precaution always Backup (File - Backup) before every data load in order that previous work can be recovered.
GDAT is virtually unlimited in size as it keeps the data in an SQLite database on disk. However the time it takes to load is a function of disk access speed and this varies from user to user. SSD hard drives will significantly improve performance.
The Relative List will display much faster if the Preference (F10) "Minimum cMs to Display in Relative's List" is set as high as possible. 30cM works well and just displays those relatives having a total cMs of 30 or more.
Next, more profiles, relatives and DNA segments equal slower performance. It is recommended to only have profiles for the immediate family of the family tree being researched. That is, mother, father, full siblings, blood related aunts and uncles. Cousins are better left as just relatives to avoid adding unrelated relative data.
Finally, if at all possible, the Preference (F10) "Enable Window Resizing: should be set to False as that will eliminate some of the processing required to allow window resizing.