...
OpenIMIS (online version) has an client (the computer) and a server (where openIMIS and its database are installed)
Hardware
For the server the minimal configuration can be found here : http://docs.openimis.org/en/latest/install_manual/minimum_requirements.html
For the client, a modern internet browser is required, there is no hard drive requirement, for the RAM and CPU, it depend of the operating system and other parameters.
Connectivity
openIMIS is a modern application using jquery library, this means the initial load for the application can take time on slow internet connection because those library need to be downloaded once
...