Did you update the parts and assemblies to 2019 as well. It will get faster once the files is updated as well. Have to do this each upgrade.
We see this every year. As much as I would like to utilize new features in a release I never switch right away. SP0 is typically not very good (I'm being nice). Personally I never install before SP3 is released even if some of my customers make the switch before I do.
It's a double edged sword, if nobody installs the new version right away SolidWorks doesn't get enough feedback to make the service packs better, but since I rely on SolidWorks to generate income for me I can't afford to waste my time working with a buggy release.
I will be happy to wait for SP3, am in absolutely no rush to make the switch. Will simply continue to do my work with the 2018 version.
I know with every release we see issues. I dont go live until service pack 2 at least. After going to a "whats new" i assumes the large assy behavior would be decent out of the box. When i went from 2017 to 2018 , I did not see that much lag with the same large file. Less bugs then than now. I will toy around with it when i have time but will not go live for a while.