Skruffy Kat had for years slow flash rate and occasionally non flashing indicators (usually at the MoT station

The tester was always commenting on them, but it was not ever given as an advisory.
In the end I got fed up with them and replaced the original relay with an electronic one of Ebay. This has a rotary micro switch than can be used to adjust the flash rate to compensate for using LED’s instead of incandescent lamps in the indicator units.
I don’t have LED’s in mine but it cured the low flash rate.
This is the one I have on the Kat:
https://www.ebay.co.uk/itm/162728580599