Though the mana cost is a bit high,
Inner Struggle should be able to shut down any creature they play / pump. You could play a couple copies in the sideboard. Since you were also asking about fast creature,
Monastery Swiftspear is an all star in red aggro decks with a lot of spells (like yours). It can also act as a nice blocker when they go all in on their big infect creature.
If you were splashing white, then you could run
Deflecting Palm, which is arguably the best tool against infect. Let them pump all they like, then when they swing for 10 infect, just point it back at them. a white splash also gives you access to
Boros Charm and
Lightning Helix, so it's at least something to think about.
Hop you found this helpful.