I was looking through Irisviel's page, and it says her NP gain rate per attack is 0.59%. This is lower than Heracles, and his charging seems extremely low to me. Is there something I'm missing?
There are a few other differences as well.
The NP charge rate is per hit, not per attack. All of Irisviel's attacks have more hits than Heracles's. (She has 3 hit Arts cards)
Like most Casters, Irisviel has Territory Creation which boosts her Arts performance. With 3 Arts cards in her deck, that helps a lot with NP charge.
Lastly, her second skill boosts her NP generation by 20 to 30%.
According to Cirnopedia, she only gets 0.42% gain per hits and usually Casters have Territory Creation Passive to boost their art cards. Iri has B rank of that passive (8% boost), so it would end up around 0.45% per hits with 3 hit counts (1.35%) which is kinda low for casters standard, so you may want to give her CE like Divine Banquet or Prisma Cosmos. But, if you can consistently doing arts chain, that won't be a big problem.