QuickBase is single threaded within an eco system of Apps which are connected. Apps are considered connected if they have cross app relationships or I believe Report Link fields also cause them to be connected.
If you have large data sets and / or large numbers of users on the same app, then planning is required to avoid casually causing apps to be connected. Using Sync tables is the main tool we have as an alternative to cross app relationships.
It's hard to say what "large" is since it really does depending on the complexity of the relationships and the use of summary fields.
It's not that all cross app relationships are automatically bad, it's just that it will cause those two apps to be treated as one large app for the "single thread processing".
If you just have 1 app which is not connected to others then the single threading is for that app.
Your actual app will live on a server used by other apps from other users unrelated to your company. They can also affect your perceived performance.