As discussed in many many threads before:
- HEVC media require 3-5 times more processing power (VC-1 is only slightly less problematic)
- ‘burning in’ bitmap subtitles (PGS, VOBSUB) require also much more processing power
- High-bitrate files, (even in H.264) require more processing power than the 2000 passmark points in this help article
So, you should keep that in mind and prepare your files accordingly:
- Avoid HEVC and VC-1 encoded files
- only use text-based (SRT, TX3G) subtitles
- if you store full BluRay rips (bitrate up to~35mbps) be prepared to have ~4000 passmark points available per transcoded stream (the additional load from bitmap subtitles and VC-1 codec not taken into account!)
Generally: If you have a choice between several cpu models, do look them up at http://www.cpubenchmark.net/cpu_list.php
Do not only look at the overall benchmark number, but also at the ‘single thread rating’. This is crucial for Plex transcoding.
When you have to choose between models with similar overall passmark score, pick the one with the highest ‘single thread rating’.
Don’t compare the performance of a desktop cpu to that of ‘mobile’ cpu’s (as built into laptops).
Also, there are i7 and i7 cpu’s 
Again: always look up the particular type of cpu (model number and clock rate).
There are a lot of Xeon cpu’s in use for Plex server which are not really suited for the task. They have a high passmark, but ‘single thread ratings’ of around 1000 points. Those cpu’s suck when used with VC-1 encoded files or when using bitmap subtitles.
This i7 cpu on the other hand, handles 2 parallel full Bluray rips with bitmap subtitles quite well:
