I consider upgrading, but Plasma 5 has not all features yet that I like in Plasma 4. Maybe if it is significantly faster I will switch. So, is it faster?

I've heard (from reviews and things around the 'net) that plasma 5 is supposed to be lighter-weight (reduced ram usage) than plasma 4, and may well feel faster/more responsive. I haven't tried it yet, so I couldn't say for sure.
You could try running VMs of Kubuntu 14.04 and 16.04 (or project neon) and compare for yourself

Much faster. Like blazing fast! Do give it a try.

I have just jumped on the KDE band wagon with Plasma 5.10 (fedora 26) and I am SO impressed with the performance of this version.

I was previously an XFCE4 user since KDE has historically too sluggish for me but I can't say enough good about this and I am a stickler for performance. If these was any lag at all I would have abandoned this 5 minutes after I installed it, but instead I have wobbly windows and a rotating and pointless (but awesome) desktop cube going.

I'm on a Dell xps 13, Intel(R) Core(TM) i5-5300U CPU @ 2.30GHz with the max of 8gigs of ram.

I'd recommend to upgrade to Plasma 5, Plasma 4 is infact slower and well it's outdated.

Just make the choice, no ones forcing you to upgrade

3 words: Yes it is!