3840x2160 subtitles lag the video

Hello.
Hard is the life of an early adopter. This time, I am having problem with subtitles. In MPC-HC options, since there was no 3840x2160 option, I set subtitles' resolution to 'Desktop', which has the same result.
The problem is that each time the subtitles are appearing on the screen, the video lags. It gets even worse with styled subtitles (ass) which can even lag the audio.
Here is a graph showing the lag on a standard 1080p film.
https://i.imgur.com/VD7rwNA.jpg

Setting subtitles resolution to 1920x1080 solves the problem, but it results in them being a blurry on my UHD monitor.
Do I have some settings wrong? I don't think my i7 4770 and Radeon R7 260x should have any problem with rendering text.
I am using K-Lite 11.1.0 x64 installation.

Subtitle rendering is actually quite complex, certainly at such high resolutions. It is single-threaded, so it only uses just one CPU core, and even a fast CPU like yours can have trouble.

Two things you could try:
1) Change the buffer size. Try sizes 0, 10, 30, and 60.
2) Use madVR as video renderer. Set subtitle resolution to 1920x1080. madVR should give better scaling quality. It won't be a sharp as desktop resolution, but maybe good enough.

The MPC-HC developers have plans to improve scaling quality of subtitles:
https://trac.mpc-hc.org/ticket/5393