From version to version there is a progress in this regards actually. Next version will have even lower memory usage. But some limits just very hard to overcome. Take note that sometimes even the simplest web pages consume dozen of megabytes of ram in the Chrome. It is well famous for this feature : )

The 250 MB memory consumption can be real. If you have something around 15 000 saved nodes. Next version will reduce this, but not on a magnitude.

My advice for now, if you have many thousands nodes (note that other extension just cannot even come close to such counts of simultaneously visible nodes) and the ram consumption is a big issue for you - expand all nodes, save the tree as a plain text web page (through Ctrl-S). 

Check if this page is rendered ok (reopen it in some tab)

Then uninstall Tabs Outliner - this will delete all its data (WARNING!!! THIS WILL DELETE ALL ITS DATA!!! RE READ THIS). Then install it again and start your tree from scratch. This will return you ram usage to 10-15MB. 

Use your saved in html tree to access  for old data (you can drag from it back to main view some individual windows and groups by the way). 

Other solution - having most of the part of tree collapsed - if items is collapsed it is not rendered in HTML, and consume very little ram. But it is actually more useful to have your old tree as a separate web page.

There is some price that need to be paid for some features. If you have a lot of data visible on screen in HTML world this is unfortunately lead to a big memory usage. In fact - if you continue to grow your tree indefinitely the ram usage will continue to grows also, you can reach any digits that you want this way. So you need decide what is more important for you - the ram, or having all the data in one tree.

I am periodically (once in 6 months actually) drop the whole tree in a file and start from scratch. Soon there will be better way for a same - it will be possible to create separate tree documents. So all the data will continue to be searchable and editable (the tree in a static HTML file is not editable by drag and drop... but editable in any HTML editor by the way)

Best.