kaspersky, bitdefender, norton 360 are considered equally good options, but the one that comes with windows is pretty good these days as well. honestly don't know if you need a 3rd party anti virus but can get one for peace of mind I guess..
hardware wise, computers start getting slow when dust accumulates in fans and they can't spin properly anymore or if thermal paste between the cpu and cooler hardens and no longer provides heat transfer. the cpu then gets hot and clocks itself down, so it doesn't end up cooking itself. pcs need a good cleanup at least once a year.
software wise, as you keep installing programs, especially ones on top of the other if they're updating themselves, you start getting errors at some point.
need to constantly update drivers as well for better performance. I recommend after purchasing your AIO to go to Lenovo's site and download the latest drivers for your model, also go to Intel's site and download the driver and support assistant.
then, as the partition windows is installed on gets fuller, it's harder to map out spaces on the drive for new uses, so it needs to move around information and that takes time. You will have a small solid state drive with windows and programs on the new pc, don't use it as storage space. Anything large you need to keep, put it on the big hard drive. after windows will have finished installing it's own updates, go to Storage in settings and clean up the old windows installation files, these tend to be large and are not neeeded.
a free, simple to use piece of software you can use once a month is ccleaner, keeps things tidy. make sure there is nothing unnecessary running at startup
best piece of advice I can give you tho is to not open any links from random emails or click on shady ads on websites. you know the ones, with the penile enlargement pills and such
don't click on those or anything that promises the moon and the sky like free iphones etc and you will keep a nice tidy pc that will continue to work fine for years.
some more helpful tips for a snappy windows experience
https://www.windowscentral.com/tips-...nce-windows-10
gl with your new purchase sir