When you talk of scalability you have to mention the App Server in which you will deploy your Portal. Even Websphere supports Liferay.
Normally Liferay is deployed on Tomcat, which has its own advantages like being open source and all. That helps an organization save loads of money.
But in case you are referring to Portal Solutions provided by IBM then its a different story. Using IBM Portal Solution is expensive compared to Liferay, and with Liferay you have the choice of using App Servers of choice(Glassfish, Tomcat, JBoss to name a few) plus Clustering and Load Balancing is there to help in Performance.
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