The topic of poor Jira performance came up yesterday, and I did some quick benchmarking of Jira cloud using the best-case scenario for performance: A tiny amount of data, no complex permissions, a commonly used form, no web proxy, no plugins, same geo region as the servers (Sydney), gigabit fibre internet(!), etc...
I spun up a free-tier account and created an empty issue. No data. No history. Nothing in any form fields. As blank as possible.
The only positive aspect is that most of the traffic is coming from a CDN that enables: Gzip, IPv6, HTTP/2, AES-GCM, and TLS 1.3. That's the basics taken care of.
Despite this, reloading the page with a warm cache took a whopping 5.5 seconds. There's an animated progress bar for the empty form!
This required 1.2 MB of uncacheable content to be transferred.
With the cache disabled (or cold), a total of 27.5 MB across 151 files taking 33 seconds is required to display the page. This takes over 5 MB of network traffic after compression. (Note that some corporate web proxies strip compression, so you can't rely on it working!)
For reference, it takes 1.6 seconds on the same computer to start Excel, and 8 seconds to load Visual Studio 2019 (including opening a project). That's four times faster than opening an issue ticket with a cold cache!
Meanwhile, the total text displayed on the screen is less than 1 KB, which means that the page has transfer-to-content efficiency ratio exceeding 1000-to-1. This isn't the animated menu of a computer game, it's a web form!
To render the page, a total of 4.35 seconds of CPU time was required on a gaming desktop PC to with a 3.80 GHz CPU. Having 6-cores doesn't seem to help performance, so don't assume upcoming higher-core CPUs will help in any way.
A developer on an ultraportable laptop running on battery over a WiFi link with a bad corporate proxy server in a different geo-region would likely get a much worse experience. Typically they might get as little as 1.5 GHz and 20 Mbps effective bandwidth, so I can see why people are complaining that Jira page loads are taking 10+ seconds!
In perfectly normal circumstances your customers are likely seeing load times approaching a solid minute.
PS: I do development, and I've avoided Atlassian products primarily because there's been a consistent theme to all discussions related to Atlassian, especially Jira: It's slow.
Stop asking your customers if they're running plugins, or what configuration they're using. Start asking yourself what you've done wrong, terribly, terribly wrong.