For my wife we've had a couple of ~$600-800 laptops over the last few years but inevitably they end up super sluggish, and I'm not exactly sure why. We're now looking for a replacement.

Use Case: Mainly browsing with many tabs, some Excel. Mostly cloud usage now.

Seems a pretty undemanding load, but current HP laptop with 4GB RAM running Windows 10 Home easily has 90% RAM used (when Firefox is using ~300-400MB with many tabs) and runs like a dog. I suspect poor I/O performance as anything involving swapping like switching apps can take tens of seconds, but the hard drive is not fragmented and raw hard drive performance seems ok. Frustrating that on a 4GB machine, just 400MB on Firefox and little else running (seems to be a pile of small Windows services), memory is maxed out.

I'm thinking 8GB RAM's going to be a sure bet, and perhaps SSD as the only storage. Worthwhile?

Is poor performance inevitable in the $600-800 on special range? What sort of money should we spend to get something that'll last a few years and still be responsive? 

Any guidance/recommendations much appreciated.