#Clojure, 81 bytes

#(doseq[[c d](map vector %(cycle[100 100 500]))](Thread/sleep d)(print c)(flush))

Loops over the input string zipped with a infinite list of [100 100 500].

(defn typer [input]
 ; (map vector... is generally how you zip lists in Clojure 
 (doseq [[chr delay] (map vector input (cycle [100 100 500]))]
 (Thread/sleep delay)
 (print chr) (flush)))

Clojure, 81 bytes

#(doseq[[c d](map vector %(cycle[100 100 500]))](Thread/sleep d)(print c)(flush))

Loops over the input string zipped with a infinite list of [100 100 500].

(defn typer [input]
 ; (map vector... is generally how you zip lists in Clojure 
 (doseq [[chr delay] (map vector input (cycle [100 100 500]))]
 (Thread/sleep delay)
 (print chr) (flush)))