I've had horrible slowness with entering and accepting transactions in Quicken (and clicking on accounts in the account bar) for over a year now. Investment transactions are slightly slower. When I mean slow, I'm talking 10-15 seconds, which is unacceptable for a web application let alone a local desktop app. Updates don't help. Reinstalls don't help. Validate and Super Validate don't show anything wrong. My data file is a decent size (1 GB), but I have a gaming laptop with a dedicated graphics card—and it's still slow.
I have extensive Classic dashboards and have them all collapsed so they don't refresh until I need them. I've optimized using all of the strategies listed that I could find.
I've managed to identify that using Quicken completely disconnected from the internet is about 20-30% faster. The only thing I can judge from this is that Quicken's internal telemetry for what features are used and how are interfering with the application.