We have a couple of custom portlet applications running inside Liferay Portal.
The solution is installed on client’s computer which is entry-level (RAM <= 1 Giga). Due to red tape, it is rather unlikely the client switches to higher-end computers in the short term.
The issue is that the applications are very slow.
What are the hints to optimize Liferay configuration (or optimize the portlet application) so we are able to run decently on entry-level computers?
Or is it a good move to switch the portlets to lighter Portlets Containers alternatives such as Apache Pluto or GateIn?
Or running a portal like Liferay on entry-level computers is not an option? And we should consider porting the existing portlets to separate standard Java Web Applications so to achieve better performance?
Compare the price of tuning, minimizing the footprint and measuring the result to the price of just 1 more Gigabyte of RAM - which you might not even be able to purchase in this size any more.
Then compare the price for porting from a portal environment into Java Web Applications: You can't even be sure that this will result in a lower footprint, as you'll have to redo quite a bit of functionality that Liferay provides out of the box. Identity Management for example. Content Management as another one. This will take time (equaling money) that might be better spent with just a new server.
For ~40€/month you can get a hosted server, including network connectivity, power and even support, that is way more capable of serving an application like this than a server the size of a Raspberry Pi (<40€ total, I've seen Raspberry Pi hosting for less than 40€ per year).
I don't know what you mean with "Red Tape", but I'd say you're definitely going for the wrong target. While there is a point to tune Liferay, I'd not go for this kind of optimization.
You're not mentioning the version you're using - with that hardware I'm assuming that it's an ancient version. Back before the current version, Liferay was largely monolithic. While you can configure quite a bit (cache, deactivate some functionality) they'll not bring drastic advantages. The current version has been modularized and you can remove components that you don't use, lowering the footprint - however, it's not been built for that size of infrastructure.
And when you're running the portal on that kind of hardware, you're not running the database and an extra webserver on the same box as well, right? This would be the first thing to change: Minimize everything that's running outside of Liferay on the same OS/Box.