PDF scrolling with keyboard is slow

[lancelot-2]

In 57.0.2 on 64-bit Windows 8.1, Text Link 6.0.1, PDF scrolling is smooth if I use the mouse wheel or drag the vertical scroll bar. But if I press the Down arrow or PgDn, it takes a whole second for Firefox to respond.

The issue goes away if I disable Text Link.

[JanecekPetr]

Same happens for Twitter, Stack Overflow, and perhaps more. Unfortunately Text Link is definitely behind this. Please help us, it really makes our daily browsing unbearable. Do you need any further diagnostic information?