Hey Stad,
This can be a challenging area, technically at times like finding a needle in a haystack.
Is there anything observed that might rule where the issue is coming from? I'm thinking Client-side issues such as hardware struggling to render, service level specific issues, webmap/app level issues, or an overall performance issue on your ArcGIS Online organisation?
For starting points I would take a look at your ArcGIS Online Health Dashboard, and ensure we are not seeing a prolonged high average usage, as ArcGIS Online will maintain a lot of health measures where requests are throttled if there is a risk of Datastore activity causing a crash - This guide here is useful for troubleshooting at your Organisational datastore level.
If things look fine at the datastore level, then I would recommend comparing your Web Maps and services. Are these all running slow, or specific ones? If you do a ctrl - f5 refresh of the Web Map/App while using the web browser dev tools, and check the network tab how long are the slowest requests taking?
It can be a subjective area, as some configurations need a lot of requests, and are fine with a 2 second query time, while others may find that unacceptable, but the key thing I'd look out for is if queries are finishing wthin this timeframe, or if they get slower and slower, occasionally on some web map configurations we can see queries take over 20 seconds each.
If you think the issue is at this level, then I would try reduce the number of requests being sent, using Web Map optimisations such as clustering, scale thresholds, aligning Basemap and Layer projections etc.
Service level optimisations may also help you, such as trimming change logs, and performing a spatial reindex if services are fairly old, and have been edited.
Hopefully that gives you a few pointers. As George said above don't be afraid of contacting support as this can be a tricky area to troubleshoot.
David
Esri UK -Technical Support Analyst
