@jgamel
You can think of workspaces as just extra tab bars to visually-only split your tabs between. The default tab bar is workspace (tab bar) 1. If you create an extra workspace, it'll be workspace 2 (tab bar 2).
Whether you have:
Workspace 1 (Default tab bar): tabs 1, 2, 3, 4, 5, 6
or
Workspace 1 (default tab bar): tabs 1, 2
Workspace 2 (tab bar 2): tabs 3, 4
Workspace 3 (tab bar 3): tabs 5, 6
, you still have 6 tabs open in the window. They're just visually separated in the latter situation. The memory usage for the tabs will be the same in both situations. There might be a little overhead for sorting them into separate visual tab bars, but otherwise, they use the same amount of RAM. So, if Opera is being too slow because of the amount of tabs you have open, it's just that and not that they're separated into different workspaces.
Note that Opera snoozes inactive tabs to save memory by default. That should apply to any inactive tab no matter what workspace they're in.
Workspaces are not a way to group bookmarks and are not a way to save sessions that close when you switch away from them and open later when you switch to them.