I have been running a Tekkit Classic server with multiple worlds for some time with no problems. I recently took that server down and started a Tekkit Lite server on the same machine for testing purposes. With the Forge version of Dynmap installed the server lags to a halt whenever I enter any area that does not already have a map portion generated. If I enter the fullrender command, even for just the flat 2d map, it starts out normal then the render time begins to increase exponentially. So I would say the Forge version of Dynmap (at least for Tekkit Lite servers) does in face use quite a bit more RAM or something else is going on because I had to uninstall the plugin to make my server usable.