I wish the updates for Spectre/Meltdown were optional on my Windows machine. They posed no real risk to my single-user gaming machine and the fixes had an unacceptable performance burden IMHO.
Where are you seeing a performance burden? The spectre/meltdown patches had no impact to game performance: https://www.tomshardware.com/reviews/gaming-performance-melt...
If lscpu still shows some offline cpus (let's say cpu 1 and 3 are offline), echo 1 >> /sys/devices/system/cpu/cpu<1 or 3>/online. Run lscpu again, and you should see all CPUs online and 2 threads per core.