Apache is very fruits under load, when many slow clients (in terms of download speed) trying to get the pages. Therefore, loaded the server, I recommend to put to him the nginx as reverseproxy. He will look to Apache for page, who will quickly work out the process, then slowly "scarmouche" it to the client, not allowing apachesolr process all the time is useless atirat memory.
Guidelines for setting up this outrage in the Internet a great many, so that the links will not lead. ;)