If by "last time" you mean the pre-WebExtension version - yes, that version was rather CPU intensive and I used a different extension (no longer maintained post-WebExtension) back then.
I haven't seen any negative performance impact as far as battery or CPU usage are concerned, but as usual YMMV.
I just gave it another try. Unfortunately, things haven't improved. `about:performance` shows a "medium" energy impact for TST. My 2015 Macbook's fans come on and stay on until I disable TST again, after which they reliably spool back down after a short delay. While TST is enabled, Firefox shoots up the list of CPU users in Activity Monitor.
Oh well, it was worth a shot, sidebar tabs are just so good when they're working well, but it's not worth the battery drain.
I'm using a bunch of other extensions and customisations, so I guess there's some dodgy interaction going on somewhere. No time to go spelunking though. I'll just use it on my desktop instead, where I won't notice the load.
I haven't seen any negative performance impact as far as battery or CPU usage are concerned, but as usual YMMV.