If it’s slower then you are used to, it may be running in “something is wrong” mode with the battery removed. The new battery may put you back where you want to be.
II don’t think it’s the laptop specs that are giving you trouble.My system is ~80% the CPU (hard to make a direct comparison) and 1/4 the RAM and it is quite happy running both Lightburn and my CAD/CAM…
You definitely want a spare computer in case this one decides to give up, but throwing money at a higher performing system isn’t necessarily going to give you the results you might be expecting.