Are drawings slow in Elevation views only? If so check if you use the Matrix Switcher shape. This shape had performance issues with a recent Visio update. We have updated the shape and the latest Elevation Shapes.vss has these issues fixed. You will have to download stecil updates in SI 2017 to get the latest Elevation Shapes.vss. In any existing drawings you have to replace this shape (Matrix Switcher) with the latest shape from Elevation Shapes.vss. This can be done via the “Change Shape” function.
Are drawings slow on pages with a lot of schematic wire connections? Make sure on Project Settings you have “Automatically audit wire connections” set to “No”. This is on Global Project Settings | Connections tab. If set to “Yes” it may take a while to check if every wire connection is valid when you activate a page.
If the above do not help can you send drawings and project (Export Project) to D-Tools Support and ask them to mark case for development and we can take a look. Also let us know your client and server versions please.