Hi Nathan,
Since you mention "it can barely serve 1 client without problems", which I can assure you is not a systemic issue with WebLink 11 upgrades, I wonder if the system may be affected by this recent issue:
The May 14, 2024 Cumulative Update for .NET Framework May Cause Reliability Issues with the Laserfiche Web Client and Laserfiche WebLink.
Long story short, a recent non-security Microsoft .NET update broke some WebLink and Web Client deployments that causes rapid chain crashes of their IIS worker process. From the end user perspective, these rapid crashes can look like performance issues because the web app launches, tries to do something for the user, crashes, relaunches, then browser fires off retries for incomplete calls, then the process crashes again after maybe getting past loading the next tile, relaunches, crashes, and so on.
We have customers where both WebLink and Web Client are affected, only one, or none. We haven't figured out exactly what causes the issues yet (or at least all of the things - there may be multiple) but are actively working on it.
I would check if the issues only started occurring after the May 2024 .NET Framework Update was applied. If so, uninstall that update per the instructions.
You should also look at the event logs on the server with WebLink. Check Windows\Application, Windows\System, and the Applications & Services\Laserfiche\WebLink (Portal) logs for any Error or Warning messages that could correlate with attempting to (slowly) load an image. Specifically check for the error message mentioned in the Support Site article linked above, but there could be other relevant ones as well.
If it's not the .NET Framework Update issue, please open a support case. Being able to "barely serve 1 client without problems" strongly suggests there's some sort of issue going on that needs to be resolved before thinking about WebLink-specific performance tuning.
That said, I do have a general IIS settings optimization PowerShell script that updates some defaults to make initial response times faster by "preloading" the web apps instead of waiting for the first request to come in. The script is named " iis-config-optimization-extended.ps1.txt" and you can find it here: https://answers.laserfiche.com/questions/193847/FormsInitial-Login-Slow#193889