Unfortunately, the problem still exists!
My setup consists of two displays: the internal display (MacBook) and an external one.
When blur is set to maximum:
both displays, Alfred on the external display: no lag
both displays, Alfred on internal display: lag
only external display: no lag
only internal display: lag
Setting blur to under 50%, there is no lag at all (no matter which setup).
Theme: https://github.com/depressiveRobot/alfred-theme-material-mac
Alfred: v3.5.1 [878]
macOS: 10.13 (17A405)