... or you can change to "Ciphers chacha20-poly1305@openssh.com" in your ssh server to use chacha20-poly1305 instead of heavier AES.
or if you haven't disable it, try changing orders in putty's Connection -> SSH -> Cipher and make chacha20 be the first one.
but Kex can be slow (or very slow) if you're using a very long DH key, ECDH key can be faster.