Recent versions of QGIS have switched to a safer approach to reading SQL Server layers, which automatically checks for geometry validity and repairs geometries when needed. This workaround is required to avoid issues with the design of SQL Server's geometry handling, where encountering any rows with invalid geometries can silently abort a request without returning all the features from a layer. Unfortunately, the workaround comes at a significant performance cost.
In QGIS 3.4 a new setting was added to allow users to manually turn off QGIS' SQL Server invalid geometry handling. Turning on the "Skip invalid geometry handling" setting under a SQL Server connection's properties causes QGIS to skip all the expensive geometry validation processing, but firmly places the responsibility for ensuring that all features on the database have valid geometries back upon the user/database administrator. This is a "use at your own risk" setting. Improperly enabling this option can lead to missing features and data loss.
This feature was developed by Nyall Dawson (North Road)