If you are working in Domino Designer with a lot of databases at once or if those databases are on remote servers, it tooks forever to build even a single xpage. Eclipse is rebuilding (or at least checking for changes in) ALL opened databases (in Eclipse they are called projects).
You can uncheck Build automatically in Project menu or use Navigator view to close unneeded dbs. Navigator is not by default part of the Domino Designer perpective, so go to Window - Show Eclipse View - Other and in category ‘General’ you can find ‘Navigator’. Navigator appears next to Data and Controls panels (you can put it somewhere else if you want) and it is the only place where you can actually close the database. Simply right-click on the database and select Close project. Or you can right-click on one database you want to left open and select Close unrelated projects and all other dbs are closed.
It is recommended to save the Domino Designer perspective to retain Navigator view in it even after restart of Designer (Window -> Save perspective as… and leave the name as Domino Designer)