What are high speed earthing switches used for in GIS?
In "my" utility these are not ever employed for auto-grounding purposes at 500 kV, as anywhere there's 500 kV there's OPGW [ "optical ground wire" ], meaning "sky wire" with a fibre optic bundle in the centre, meaning digital teleprotections can be employed.
Aside: for a while there was a site at a remote tapped location that used vacuum auto-grounds @ 230 kV, but that was only an interim solution until OPGW was installed on the circuit through that site.
We do have one location @ 115 kV where an SF6 fast-acting ground is used as an auto-ground, but other than for that site, any of the auto-grounds we still have in service are all air switches.
All that being said, within my utility SF6 FAGs are almost exclusively employed for the purposes for grounding off-potential circuits for work protection, most commonly in situations where relatively high residual or induced voltages may still be present on that equipment despite its having been switched out of service. The FAGs are typically high speed both when being applied and removed; however there are some legacy ones that are quick on, but slow off, and specific instructions / operating restrictions are in place to ensure proper sequences are followed to preclude these being damaged, these being based on studied of the capabilities of the grounding switches at the various sites connected to that circuit, spelling out in great detail the sequence in which grounds are to be applied and removed, including, when needed and only as a last resort, the grounding or un-grounding of a circuit via circuit breaker, a most time-consuming but necessary procedure.
CR
"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]