Mum has an old pre-built Win 7 Acer (upgraded to Win 10) and is in desperate need of more speed. She won't let me build her a new one

I'm doing the next best thing and upgrading the CPU, and moving from HDD to SSD (after finally finding the SATA ports hiding behind the HDD). The question was also asked last year:
https://community.acer.com/en/discussion/602849/gen-3-i5-cpu-for-aspire-x3990?breakffcache=b1cd5 but I'm hoping someone can offer a better CPU than the i5-2500, which is only a little bit faster than her current one (i3-2120). I know the SSD will make the biggest difference but a 22% improvement is not worth the expense. I was hoping to get closer to a 100% increase in effective speed.
Will a Samsung 870 QVO comfortably sit inside the case or will I have to find a way to secure it?