Okay, I've ran some tests and noticed that:
My antivirus is acting up whenever I'm using vanitysearch, it's not detecting anything but consuming a lot of resources.
Much higher than what Vanitysearch is using; so I've included it to my AV's "exceptions list".
And it worked!

Somehow, vanitysearch isn't using all of the cores or even processing power when using that command, there should be something wrong.

Whatever the reason is, I've tried to use 1 thread and it resulted with a slightly slower rate (just about -10%).
So, try to run 3-4 instances of Vanitysearch running with the same command but pointing to different output files.

Creating batch files is a good shortcut to do this, eg.:
instance1.bat
instance2.bat
instance3.bat
...And so forth

Each instance will be slower than a solo instance but overall number of generated addresses and keys could be higher.

Don't know but your previous speed doesn't sound right for an old i3 CPU from 2012 (third-gen maybe) when searching for '1*'.
You'll likely get 1000-2000 per update of "Found" at best (based from my PC w/ i3-4130 procie).

Other than the additional -c which is unnecessary for a wildcard prefix, I can't find any issues with the command you're using.
If you previously used an older version with faster result, then downgrade to that.

The weird thing is, your processor can search for Million keys per second, while it can only "no-prefix-search" random address & key pair @ thousands per second.
Perhaps it's not meant for that use-case.

If it's not a hardware issue, my guess is: Vanityseach might be struggling to write on that output file due to its current size.

Have you been using the same 'output.txt' file since the start?
If yes, try to use a new text file like "-o output2.txt" and see if it will speed-up.