Acrobat Reader and Pro are bloated. Apple's Preview runs circles around it and that doesn't make sense being Adobe's own Reader and file format should work flawlessly (by now). Pro can do more things like make PDF forms. But ever since Acrobat 5, it's become an overly large Application that runs unacceptably slow.
PDFs were originally intended for books. Later functionally was added for PDF forms, archiving records, printing and prepress. They are doing too much with it and there about almost 20 versions of the PDF file format if your include PDF/X, PDF/A, PDF/E, PDF/VT and PDF/UA in addition to PDF 1.0, 1.1, 1.2, 1.3, 1.4, 1.6, 1.7 2004, 1.7 2006, 1.7 2008, 1.7 2009, 1.7 2011, 1.8, and 1.9.
It's a shame that Adobe can't tame this its own wild beast. Adobe PhotoShop CC and Illustrator CC are excellent. Indesign CC is good with the exception that its Alpha Channel form of PDF transparency is not compatible with Firefox's builtin PDF plugin. Indesign's drop shadows will come out as boxes in both Dropbox and Firefox's internal PDF viewers. As a stone age workaround, you can use PDF 1.3 to flatten the file, but then you face unwanted lines in the web view. InDesign drop shadows have always worked in the inverse of Illustrator's drop shadow transparency which doesn't make any sense with software coming from the same vendor and both were created from scratch (to the best of my knowledge the source code to these two apps were not bought from another company). Adobe needs to get a grip on PDF. It has been too long.
Adobe Indesign was designed to become a QuarkXpress killer. But they lost sight on how its transparency should work and things like Rasterize and Keyline view were omitted. So if you need those additional tools, you have to copy and paste the elements into Illustrator. Turn on PDF in Adobe's clipboard past preferences , do you edits in AI then copy them to InDesign via the clipboard. Waste of time to get decent looking online PDFs from InDesign. It is best to not use Acrobat and InDesign and just use Preview as your viewer or Firefox, and use Illustrator to create decent PDFs that have transparency.
One trick is to drag their preferences files out onto the desktop and restart the apps - they will build new ones, though you'll have to enter your settings again.
You might check if these are taking App Naps - look in Activity Monitor.
Best Answer
It is more than likely that the increased demand on the GPU (processing and RAM) by supporting both the external monitor and the MacBook Pro screen while using Skype or Zoom causes the MacBook Pro to lag. Since you mention that the GPU is rather old and has only 1 GB of RAM, this is probably the problem.
Skype may give you the ability to lower the video quality and resolution, putting less demand on the GPU. If you can still Skype with the external monitor disconnected, this may be the best solution.