How much memory is on your machine? I can have 50 tabs open without a problem on my MacBook Pro with an i5 and 16GB of RAM, but on older hardware I can't have more than a handful of tabs without starting to run out of resources.
RAM use varies from about 1GB to 2.5GB. It slows down above that, which you can only solve by restarting the browser.
I bought another 8GB so I reckon my 2011 machine will be good for another few years. It should last a decade, really.