Performance poor in webworker using data uri pngs.

[jsoo1]

Bug Report

I found #855, #264, #269, and #1081. All of which seem related to the problem I am having.

Description of the problem

I want to make a pdf out of full page pngs which are data URIs. I want to run in a webworker and have compression so I cannot say which of the afforementioned issues is really to blame.

The problem is that performance is very bad. The problem seems to scale with number of pages. One page is "fast" (~1.5 seconds). Two pages is much slower. I recorded several data points (as many as patience granted).

These data were generated using the code below.

pages	time
1	1561.501953125ms
1	1581.448974609375ms
1	1658.505126953125ms
2	175932.03173828125ms
2	173780.81176757812ms
2	178272.82006835938ms
3	235536.85009765625ms
3	234518.7412109375ms
3	233301.33813476562ms
4	237735.68701171875ms

Code sample

const PDFDocument = require('pdfkit/js/pdfkit.standalone.js')
const blobStream  = require('blob-stream')

onmessage = ({ data }) => {
    const dpiScale = 72 / 300
    let pdf = new PDFDocument({ autoFirstPage: false })
    let stream = pdf.pipe(blobStream())
    data.forEach(c => {
        pdf.addPage({
            size: "letter",
        }).image(c.dataurl, 0, 0, {
            width: c.size.width * dpiScale,
            height: c.size.height * dpiScale
        })
        delete pdf._imageRegistry[c.dataurl] // <- Got this from #1081
    })
    console.time('data')
    pdf.end()
    stream.on('finish', () => {
        console.timeEnd('data')
        let blob = stream.toBlob('application/pdf')
        postMessage(blob)
    })
}

Your environment

• pdfkit version: 0.10.0
• Node version: N/A
• Browser version (if applicable): Google Chrome Version 80.0.3987.149 (Official Build) (64-bit)
• Operating System: macOS Catalina 10.15.4

[jsoo1]

When removed from the webworker, times reduced greatly.

new Promise((resolve, reject) => {
    try {
        const dpiScale = 72 / 300
        let pdf = new PDFDocument({ autoFirstPage: false })
        let stream = pdf.pipe(blobStream())
        conversions.forEach(c => {
            pdf.addPage({
                size: "letter",
                layout: c.size.width > c.size.height ? "landscape" : "portrait"
            }).image(c.dataurl, 0, 0, {
                width: c.size.width * dpiScale,
                height: c.size.height * dpiScale
            })
            delete pdf._imageRegistry[c.dataurl]
        })
        console.time('pdf')
        pdf.end()
        stream.on('finish', () => {
            console.timeEnd('pdf')
            resolve(stream.toBlob('application/pdf'))
        })
    } catch (e) {
        reject(e)
    }
})

pages	time
1	1488.033203125ms
1	1233.98486328125ms
1	1121.4951171875ms
2	2580.294189453125ms
2	2574.9169921875ms
2	2376.490966796875ms
3	3750.450927734375ms
3	3778.68798828125ms
3	3539.64892578125ms
4	5161.190185546875ms
4	4778.90380859375ms
4	6695.65380859375ms

[jsoo1]

The same performance happens with pdfkit version 0.11.0