Hello Conred55,
You must evaluate on your specific system!! Even though you and I may have identical systems we will most likely not have the same experience. Software compatibility is a critical factor.
RAM usage is not so important, especially if you have more than 4 GB of RAM. My system has 6 GB and I have never "consumed" more than 75 % - even with many...and I mean many (> 10)...applications actively running.
If you have W8.1, then you will generally have good boot time with hybrid boot enabled. I assume you mean fast time to active desktop. This is more dependent upon how many startup items you have enabled. Tweaking your startups can dramatically improve things. However, some AV, like Comodo is slow to get to active desktop, but relatively light on resources...whereas Webroot is fast all the way around...
Scan times will be dependent upon many factors, but mostly by how many applications/files you have on your system and - most importantly - scan profile used - Quick, Smart, Full (Deep) and Custom. Most AV offer these scan profiles, but call them by different names. Processor is also a factor.
On my AMD A8 6410 system, Emsisoft Internet Security 9 Full (deep) system scan takes almost two hours. It's not that EIS 9 is slow, it is that my system has low-end processor and is loaded-down with many, many, many...many (> 350,000) files.
All deep system scans are processor intensive - from Avira to Webroot.
Very fast default scan time (uses less thorough scan profile)...only Webroot, very light on resources...only Webroot, Webroot protection...I think perhaps not so spectacular. Trend Micro has very long scan times. ESET is light on resources. Kaspersky solid protection, with average resource usage.
One thing about security suites...the more options, the greater the complexity. HIPS - not for novice/amateur night.
My best advice is to try at least two or three on your system...evaluate them to make an informed choice. That's why there are trial periods available. Each one has its own personality or flavor, if you will...and keep an open mind.
Otherwise, you might be trying to play Ping-Pong with a bowling ball, or is it bowl with a ping pong ball? You'll drive yourself nuts and not be happy if you make a commitment and fork over your $ before trying several out...
Check out AV-Comparatives...they rate all usability areas...detection, resource usage, etc.
http://www.av-comparatives.org/comparatives-reviews/
Watch youtube videos of evaluations...
You also need to consider support. Kaspersky - India, Bitdefender - forget it, Comodo - US, ESET - Czech, Avira - Germany, Trend Micro - US, Emsisoft - International, English, forum only, etc. Most support from all vendors is through online "Help Tickets" unless you want to fork over more $ for phone support.
Also important is how well AV removes infections. Some AV, like Bitdefender and Kaspersky handle this task better than average...others you will have to use 3rd party utilities under the advice of someone experienced in malware removal.
If you are average user who only surfs to youtube and downloads very little...then all that you mentioned will be more than sufficient. If you download a lot using torrents, file exchanges, warez sites, porn sites. use keygens, etc - or - if you are Enemy-of-the-State No. 1, then no AV will save you. Surf far enough, and long enough, and the system is going to get bit. It's inevitable.
Hope this helps,
hjlbx
PS - My personal choice, obviously, Emsisoft Internet Security or Emsisoft Anti-Malware.