、。
If you trace it back, it's because you, Ken, are so slow on the uptake!
Literal
Origin-[topic-は][quotative-と]-say, Ken-[diminutive-ちゃん] [subject-が] slow-[contracted-鈍ちん] is [nominalizer-のが] bad [because-だから][confirmation-ね](elongated-っ).
Casual accusatory speech. 元はと言えば ('if you trace it back to its origin, originally') is a set phrase for assigning blame to a root cause. 鈍ちん is a casual/childish slang word for 鈍い ('dull, slow-witted') — 鈍 + the diminutive ちん. Combined with ~ちゃん (the affectionate diminutive), this gives a pouty, teasing, familiar tone.