Stopped using third-party anti-virus somewhere after the initial upgrade to win8.1 when so-called one of the best AV "bitdefender" ultimately slowed down my pc and gave a pretty nice havoc to the performance. Once upon a time my pc was infected with virus and AVG didn't help until the virus decided to spread into system files. So far, never have any issues without a third party AV. Can't say how good, but windefender is reliable, in terms of up-to-date definitions and RT scans.

(Off-topic: Not condemning any 3rd party AV in my post but only certain ones. Some longest and most reliable 3rd party AV I've used are Avast and 360.)

I have 16GB RAM and an i7 quad-core processor. The slowdown wouldn't consume only RAM and CPU, but also the hard drive's (disk) performance when the disk usage would be crammed full by the third-party AV (especially bitdefender, avast had no such case). Not SSD though.

actually you may want to hold off on ssd upgrade for now.

intel and micron had come up with a technology called x-point which is set to thrump nand which is used in currenct ssd flash drives.

it wins it in so so many levels.....
1. price/peformance/endurances, it wins on all these levels. especially interesting, it's now a very strong contender not only in regards to storage, but also against dram for computer memory.

2. because xpoint does not use transistors, it is capable of having 1000 times the endurance compared to a nand ssd

3. latency is in the nano seconds rather than miliseconds. basically it's just as good as dram in that regards.

4. more storage space was able to be crammed inside. so vastly larger storage spaces in the terabytes is possible.

5. the cost to produce this is also cheaper to boot despite so many gains on so many levels where it matters.
first products due in 2016 so wait till then.