It's been quite some time since I last used MBLs with their stock firmware, but I remember that they always ran notoriously low on RAM - 20 MB or less free RAM was usual. A lot of this is cached, I am assuming by the Samba server, but there's a reason why WD have decided to include and use a swap partition. So, no, I don't think that's your issue.
Also, yes, I witnessed MBLs getting slower over time. I have no idea what is degrading and why, and I wouldn't want to guess, but I can confirm the problem. My MBLs started out at ... I want to say 80, 90 MB/s, and in the end after a good 6 or 7 years of use, they maybe did a third of that at best? Again, it's been some time.
The tiny NAND chip is only 512 kb and only holds the bootloader. I have never seen an MBL's NAND fail, it is only used for a second at boot after all. But it's entirely possible that yours is failing, I have seen MBLs fail in weird ways. MBLs are going into their teenage years now, after all.
Anyhoo, this being the OpenWrt forum, I can, from own experience, recommend running OpenWrt on an MBL. It runs leaner (about 200 MB RAM free after boot), almost as fast as stock, and way more "standard" than the highly customized Debian of the stock firmware. And obviously it's a completely current system, not one that is almost a decade old.
However, it's not as "plug and play" as the stock firmware, not even close. You will have to get familiar with setting up the base system and every single service you want to run on it. And even though a lot of it is written up and this forum is usually quite helpful, don't expect much handholding. And then there's the issue that you can not "convert" a drive, you will need to transfer all your data to another disk first because the disk will need to be reformatted. The wiki page (which, full disclosure, I wrote a major chunk of) is a start if you're interested.