I want to be aware of performance related issues with SharePoint 2010 while I'm designing my solutons. What's the best place to look up?
SharePoint performance is a very broad subject. A good source of information is the various whitepapers Microsoft have published, but it can be heavy reading. If possible do some prototyping. If performance is an issue:
1) Use the logging database (configure health and data collection in Central Administration->Monitoring->Reporting->Configure usage and health data collection) and use administrative reports to locate the slow pages(Central Administration->Monitoring->Reporting->View administrative reports).
2) Enable the Developer Dashboard for the pages you are interested in (this is not just useful for developers, despite the name) to pinpoint the source of problems, e.g. slow web part on page.
3) You can also use web analytics reports to identify pages that are giving users problems.
A good starting point for monitoring is TechNet.