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This sounds a very long time. Normally SQLite is ok at this stuff, sometimes as slow as 250KB/s, but rarely that bad. It could be due to abnormally high drive latency (which could be due to it being mechanical and over a usb, and/or it being very very fragged), or your machine being very tight on free memory (so your OS's disk cache cannot populate, or perhaps the drive itself is having trouble.
Were you able to run a program like CrystalDiskInfo? Is the drive itself healthy?
Run time normally scales with file size, so if your client.mappings.db is 20-odd gigs, it could be several days to run. This sounds ridiculous to me, so I would suggest you cancel the job and figure out what is wrong with the connection to the drive first. It would likely be easier to copy the file to your system drive with sqlite3.exe, especially if it is an ssd, and run the integrity check there.
Please let me know how you get on.