Attach (recommended) or Link to PDF file here:
Problem PDF: [wikipedia-select-net.pdf](https://github.com/mozilla/pdf.js/files/4922949/wikipedia-select-net.pdf)
Good PDF for contrast: [Wikipedia-chrome.pdf](https://github.com/mozilla/pdf.js/files/4922950/Wikipedia-chrome.pdf)

Configuration:
- Web browser and its version: Chrome 83.0.4103.116
- Operating system and its version: MacOS 10.14.6
- PDF.js version: 2.5.207, discovered in 2.1.266
- Is a browser extension: No

Steps to reproduce the problem:
1. Use attached pdf "wikipedia-select-net.pdf" above or generate your own from https://selectpdf.com/
    * This pdf was generated from selectpdf.com. For contrast, a PDF generated using the chrome native HTML to PDF converter is also provided; this pdf has no slowness issues.
2. Open PDF in the demo, scroll around, see that the PDF is slow to load

What is the expected behavior? (add screenshot)
PDF pages should render quickly for PDFs created via Selectnet. The load time should more closely resemble the Chrome PDF.

I am filing this as a pdf.js bug because Selectnet PDFs are able to render quickly in other applications, e.g. Preview or Chrome

What went wrong? (add screenshot)
PDF takes excessive time to load

![selectnet-slow-render](https://user-images.githubusercontent.com/35852738/87507844-19b74500-c63c-11ea-85c5-642bff3040c1.gif)

Related to the render time, is the excessive amount of memory consumed by the selectnet PDF. Example of memory comparison between selectnet and chrome PDF (see tabs for title)
![image](https://user-images.githubusercontent.com/35852738/87508490-8121c480-c63d-11ea-8577-1654df59d201.png)
My coworker found that:

>  the textLayer HTML element for each individual page contains the full text for the entire document, but with the positioning such that text elements not for that page are outside the viewport of that page. This is not the case for the PDF generated from Chrome. This would appear to explain the excessive memory use while viewing these PDFs because if you have an N page document, then the full content of those N pages is included in the textLayer of each of those N pages leading to an N^2 growth in memory consumption.

Link to a viewer (if hosted on a site other than mozilla.github.io/pdf.js or as Firefox/Chrome extension):

cnt glyphWdth = [];
              let j = frtChar;
              fr (let  = 0,  = wdth.length;  < ; ) {
                glyphWdth[j] = th.xref.fetchIfRef(wdth[]);
              }
              newPrperte.wdth = glyphWdth;
            } ele {
              newPrperte.wdth = th.buldCharCdeTWdth(
                metrc.wdth,
                newPrperte
              );
            }
            return new Fnt(baeFntName, null, newPrperte);
          }
        );
      }
    }
     Accrdng t the pec f 'FntDecrptr'  declared, 'FrtChar',
     'LatChar' and 'Wdth' huld ext t, but me PDF encder eem
     t gnre th rule when a varnt f a tandard fnt  ued.
     TODO Fll the wdth array dependng n whch f the bae fnt th 
     a varnt.
    me = decrptr.get("FntName");