Ask HN: WebEx alternatives? 16 years ago
If you don't need video and is looking for a similar hosted solution like WebEx, I say go with GoToWebinar. They can support larger groups, even up to 1,000.
If performance has been an issue in the past, maybe you should consider self-hosted products. Since its on-premise, you'll have more control over it. A good one is RHUB (http://www.rhubcom.com).