Try quitting all other background applications to see if this makes a difference.
I use my iPad Pro with Astropad Standard connected via USB 2.9 to a 2009 iMac with 16 GBs of RAM, with no other applications running in the background, and Astropad is working pretty well for me.
I am using Astropad and my iPad Pro with Photoshop CS4, Painter 2016-17, Sketchbook Pro and Clip Studio and Medibang Paint.
I find Astropad has been working best with Painter and Clip Studio. I use older version of Photoshop and Sketchbook Pro, and find it’s not a perfect experience, either, but it seems to be a better experience than what you’re describing.
It may very well be your MacBook.
8 GBs of RAM is not a lot of RAM when newer versions of the Mac OS require more RAM also,
In addition, MacBooks tend to use integrated GPUs instead of discreet GPUs meaning your MacBook is using more physically installed RAM for the GPU VRAM.
This leaves very little RAM for any applications, never mind heavy resource intense image editing/drawing apps.
Also, a 128 GBs SSD drive is NOT very large.
If using heavy resource drawing/image editing apps, some apps will respond better of you purchase extra external drives to use a scratch disks locations.
Depending how much data space is left on that SSD drive, you may not have enough free storage space on that drive to accommodate all of the extra data crunching that most heavy resource image editing/drawing/painting apps need.