I've shot RC with my Fuji X-T3 using both the XF 50-140 and a Canon EF 300/4 on a Fringer EF-FX Pro adapter. PDAF on the sensor helps.
DSCF0553 by
smerrick, on Flickr
DSCF0486 by
smerrick, on Flickr
DSCF0327 by
smerrick, on Flickr
DSCF0253 by
smerrick, on Flickr
DSCF2716 by
smerrick, on Flickr
Floating down the straight by
smerrick, on Flickr
DSCF2650 by
smerrick, on Flickr