Double letters.
Words packed with double letters: ll, ee, ss, oo and the rest. Type them as cleanly and quickly as you can for 30 seconds. Afterwards you see what each double costs you and which ones you drop.
strugglefuzzystilloffcoffeesuddenmirroraccountschoolcorrectgreenpatternagreeupperbeginningaffordmirrorfollowofferwellarrivesittingbutterribboncottonfreeschooltoolhappenstuffarrivebeenbuttonlooksuccesscottontoooccurfizzcorrectfunnycommunitybeginningtallpossiblesupplysaddlesuffereggribbonbeenfunnyallowapplycottoncarrybuttonsoccerpullbuzzwoodhiddenapplyconnectcommunityoffbilladdressoddsoccerpassedcarrybobbyofficerabbitswimmingstaggerbosssoccercommonkeepmiddlesorrystafftunnelmommystuffluggagearrivedinnerballriddleschoolstaggerribbonoddaccidentfunnysuccesssupplycoffeepatternrubberapplypullrabbitgoodaddresscarryhiddenballoonpasssittingcommunitycorrectdinnerbiggeracceptarrowdaggerstillfunnyhobbymessdummyofferbubbledifficultwellsupportwillpatternallowsummerstrugglelookoffneedagreedifficultsoonsunnyeggbossfuzzycannotbobbyballbiggerpatternguesshappenbottlebillsupplysaddlefoodofftennishobbybutterdifficultdinnertoofuzzy
About 1 minute in total. Stop if your hands start to ache.