Those are 2 browser Javascript benchmark I generally use:

Mozilla Kraken
and
SunSpider

Post your benchmark results mentioning your CPU model and Browser used.

My turn: Intel i3-7100u with Firefox 81

Kraken: 1414.8ms
SunSpider: 285.6ms

Firefox 83.0a1 (2020-09-23), R7 1800X

kraken 1780.0ms +/- 6.2%
sunspider 0.0ms +/- NaN%

1190.9ms +/- 3.6% Firefox 82.0, 20201020152236.

Operating System: Fedora 32
KDE Plasma Version: 5.18.5
KDE Frameworks Version: 5.73.0
Qt Version: 5.14.2
Kernel Version: 5.8.16-200.fc32.x86_64
OS Type: 64-bit
Processors: 4 × Intel® Core™ i5-3570 CPU @ 3.40GHz
Memory: 11.6 GiB of RAM

Sunspider: 222.0ms +/- 1.7%

CPU: Ryzen 3900x

Vivaldi 3.5.2080.3

Kraken:
Total: 748.1ms +/- 0.3%

SunSpider:
Total: 205.1ms +/- 0.6%

Firefox 82.0

Kraken:
Total: 669.6ms +/- 1.6%

SunSpider:
Total: 131.3ms +/- 3.0%

i3-4030u firefox 82.0

kraken
2114.5ms +/- 2.1%
2034.7ms +/- 1.1%
2079.2ms +/- 1.3%

sunspider
336.7ms +/- 2.4%
336.3ms +/- 2.5%
337.9ms +/- 3.2%

Ryzen 1600AF on Firefox 80.0

Kraken: 806.1ms
Sunspider: 165.4ms

On Palemoon 28.13

Kraken: 1046.6ms
Sunspider: 305.4ms Not that it matters anyway but out of curiosity are you running on low power mode/underclocked CPU?
I kinda expected the 1800x to sit around 900 to 1000ms at stock clocks on Kraken.

Edit: Oh I just noted that you are on nightly build, right?

Normal clocks. Seems I hit about 3.7GHz at most during the test. Also seems like it's frequently changing which core gets loaded most. I wonder how much that kind of scheduling hurts perf..

Yes I'm on nightly.

Ryzen 7 2700x (normal clocks)

Watefox Current 2020.80:
Kraken--------------------------826.0ms +/- 2.4%
SunSpider----------------------166.2ms +/- 4.0%
JetStream2---------------------95.569

FireFox 82.0 (32bit)
Kraken--------------------------848.6ms +/- 1.6%
SunSpider----------------------167.6ms +/- 4.6%
JetStream2---------------------95.860

Firefox 82 on clean profile, still R7 1800X:

kraken: 1019.8ms +/- 0.9%
sunpider: 188.4ms +/- 2.8%

Ryzen 1600AF somewhat modified on BIOS for low power on Windows 8.1, using Firefox.

Balanced power mode
Kraken 873.2ms
SunSpider 277.9ms

High perfprmance
Kraken 880.2ms
SunSpider 191.2ms

Low power mode
Kraken 2282.7ms
SunSpider 621.7ms

On low power it does indeed feels very sluggish while browsing.

For some fun, here are the results of a tablet powered by Intel Atom Z8350, on Firefox 81.

Kraken 6609.8ms
SunSpider 1066.7ms

Talk about slow...

I rarely bother updating browsers, so:

MacOS 10.14.6, i7-4790K:

Kraken:
Safari 12: 701.4ms
Firefox 69: 785.5ms

SunSpider:
Safari 12: 93.5ms
Firefox 69: 142.1ms


iPad Air (2013), 1.4GHz A7, iOS 12:

Kraken: 2380.0ms
SunSpider: 432.0ms

Is even worse on computers that use flash drives, like those found in low end laptops etc, consistently using the EMMC as swap/page file wears the storage quite fast and turn those devices into snail speed, needing minutes to boot and open a web browser.

To celebrate the Core 2 era, here is a Core2Duo E7400 on Palemoon:

Kraken 2221.3ms
SunSpider 431.2ms

Is surprising how well the system holds today for general browsing but it has a lot of trouble decoding video with the integrated chipset Intel G33 GPU. Adding a old and simple ATI Radeon HD5450 makes the system much more enjoyable and capable to stream a movie.