Ignore it, its the cost of Free, promotions like this for other avast paid products. This probably came after running the Smart Scan (NOT) ?
For me this really is a waste of time, I would never purchase under these circumstances.
Whilst there may well be things slowing your PC (I feel it is misleading), that has to be in the eye of the beholder, e.g. does your system feel slow ?
If it does there is a myriad of things you can do yourself to help with that, without having to purchase a program to do it.