Thanks to everyone for the replies.

I do mostly just turn it on when I need to perform a transaction now. The slowness is a problem even for the time that it needs to do that. An online wallet would solve the problem all right but that defeats a lot of the things that made bitcoin attractive to me.

For the moment electrum looks good. I might dig out an old HD to dedicate to bitcoin. Maybe an SD card will do. A transplant for long-term health after the emergency CPR!

I imagine the problem will only get worse as bitcoin becomes more popular. It's a bit worrying for the future of bitcoin. Will the cumulative burden eventually overwhelm even hi-spec dedicated bitcoin servers?

More people that I talk to now already know about bitcoin, and it's getting to the point where I can basically assume that anybody I meet that knows about Linux also knows about bitcoin. I hope the system doesn't get overwhelmed.