Dorico becomes very slow after a while...

Yes, please do send me your project and we’ll take a look. Please also do Help > Create Diagnostic Report, and attach the zip file saved to your desktop to the email as well. If you’re on Mac, it would also be useful if you could attach a system profile: choose About this Mac from the Apple menu at the top left-hand corner of the screen, click the System Report button, then choose File > Save Report, and attach that report to the email too.