Sadly, the Raspberry PI 3b Plus is the slowest PI available out on the market currently. There's are others which offer USB-3, 4GB of RAM as well as eMMC; I'll cover them later.
I current run an Emby server on a Raspberry PI 3b Plus, with a 32GB Class 10 SD, a Seagate 5tb Personal Cloud NAS, and use Xbox One and Roku as clients. I run DietPI as the OS; it's a stripped down version of Debian for PI. The other nice thing about this is that during the software installation portion, it has Emby listed as a package and will install Emby 18.104.22.168 for you and configure it to work properly (creates the Emby user/group, etc). I haven't had any issue upgrading Emby to the latest version after installing this package. I used this as a guide to setup and configure DietPI and Emby.
In testing on my PI3bP, you can't play a RAW MKV file on Emby; plays for about 1-2 seconds, then buffer for 5-10, play, buffer, etc.
Also, if you pre-encode your movies, which I do (using Handbrake and H.264), it'll work fine. When I encode, I configure them for H.264 and constant quality set to 20, and for audio, I go ahead and tell it to auto-pass-through the DTS HD 7.1 as well as add an AAC (avcodec) channel configured for 192k.
In testing, I attempted a movie recently for H.265 and like the MKV, the PI would not play the movie without constant buffering. Appears as though it can't handle this.
So, I began looking at other PI type servers which could handle H.265 and two came up. One is the RockPro64 and the other is the Odroid XU4. In looking at testing and reviews, I'm leaning toward the RockPro64 and sticking with DietPI as it's OS.
I hope this helps and if you have any other questions, feel free to respond. Thank you..