Abrowser lag in Trisquel 5.5 when using fallback session

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Hi

I recently installed Trisquel 5.5 on a system with this partition scheme:

/ on /dev/sda1 (SSD / solid state disk)
SWAP (9GB) and /home (~1TB) on /dev/sdb1 and /dev/sdb2 respectively
(SATA 7200 RPM disk)

I am using home directory encryption.

When using Gnome "classic" (fallback), Abrowser lags terribly and I
experience pauses / freezes when scrolling.

Gnome panel works well without noticing such problems.

The browser profile is on the SATA 7200 RPM disk, its cache is running
from /tmp which itself is mounted in RAM.

What could be causing this? The chipset is i5 680 (sandy bridge),
Intel drivers are loaded correctly - and this works well in Trisquel 5.0.

I'd appreciate any hints on resolving this, I prefer to use the
fallback mode.

Cheers,

Fabian

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