You say that you've cleaned junk files on your MBP, how did you do that? Simply moving certain files to the Trash actually cannot release space on your Mac. Thus, your 15' 2019 MacBook Pro starts lagging even on light tasks.
To free up more space on your Mac, you need to delete many large files on your Mac, such as useless downloads, app installers, unwanted apps, videos, etc., and then empty your Trash. Or else, the available disk space remains unchanged.
If you don't want to waste time to find the scattered useless files on your Mac, you can use the Clean Junk function of iBoysoft DiskGeeker.
- Free download, install, and open iBoysoft DiskGeeker.
- Select the startup disk (Macintosh HD - Data or macOS - Data) and then click Clean Junk on the right-side toolbar.
- Wait for the program to finish scanning the junk files on your Mac.
- Select the recommended junk files and click Delete to permanently remove them from your Mac.
Also, waiting for videos is not light work. It requires a lot of work from both the CPU and GPU. Your MacBook Pro becomes overheating may be caused by the blocked air vents. Hope my suggestions can give you some help.