Virtual columns and performance

[Mahesh]

A couple questions on virtual columns and its impact on application performance:

• Are all virtual columns values i.e for every table and every row calculated at app initialization? or is there some “lazy” calculation algorithm which calculates only when the table or row is first accessed?

• Any best practice recommendations on number of virtual columns for a table or for an app, beyond which performance could be impacted?

Interested in reading any info if this has been discussed elsewhere.
Thanks!