Steps to reproduce:
join MC 18w15a
press F5
start walking and strafe from left to right and from right to left
Video (60FPS) - laggy capes in 18w15a
Original video (120FPS) - laggy capes in 18w15a but 120fps
Environment
Windows 10 x64 Pro
Bundled Java runtime
NVIDIA Drivers v391.35
PC:
AMD Ryzen 1600
MSI GTX 1060 GAMING X 3G
8GB RAM HYPERX FURY BLACK
A lot of bugs with cape have been reported since MC 1.8. Cape Floating when you sneak, when you sprint... Shadow bug. Example, try to take an arrow on your character and your cape will have some shadow bugs.
Shadow bug:
https://mega.nz/#!M7BS2SgT!8c5mrMbkfaM1C2TK6GytPYi1xw2gXCx49HHuPaF7Ztc
Fly/sprint animation bug and cape floating when sneak:
https://mega.nz/#!VqQiGSpb!k1gmPrMp5-ua671ttsIdDRAUfZHQCkNXbuoj41tbNi0
My opinion is that bug will be not corrected like other bugs with the capes.