After we switched the session handler from 'database' to 'none' our sites were impossibly slow (sometimes 10 minutes) in the /administrator area. Front end of site loads fast as usual.

We are forced to switch back to 'database' and pray that Joomla is now secure again in this regard (with Joomla 3.4.8)

Would you strongly recommend that we switch back to 'none'?

Is there a way to keep the back-end fast with 'none'. The main problem for the slowness, it seems, is that the admin checks all extensions every time you login. (Our sites do not have a lot of extensions.)

With session handler set to 'database' you can easily interrupt the checking and go directly to another page without waiting. When set to 'none' you have to wait until all extensions are checked, making the /administrator impossible to work in.

Thank you

This problem is most likely a problem on the Hoster side.
I use None for every website, for some years now.
Even on websites with 10.000+ visitors a month and many extensions, I don't have any problems with speed.

Still, you can use Database as Session Handler if you want.
Always be sure you use the latest version of Joomla! and the extensions and never use extension which are on the Joomla! VEL list.