É possível que você tenha que dividir a carga dos 20 IPs de saída.
Informações podem ser encontradas aqui. Link
Na sua configuração do squid, algo assim deve funcionar ...
acl r_20 random 1/20
acl r_19 random 1/19
acl r_18 random 1/18
acl r_17 random 1/17
acl r_16 random 1/16
acl r_15 random 1/15
acl r_14 random 1/14
acl r_13 random 1/13
acl r_12 random 1/12
acl r_11 random 1/11
acl r_10 random 1/10
acl r_9 random 1/9
acl r_8 random 1/8
acl r_7 random 1/7
acl r_6 random 1/6
acl r_5 random 1/5
acl r_4 random 1/4
acl r_3 random 1/3
acl r_2 random 1/2
acl r_1 random 1/1
tcp_outgoing_address x.x.0.20 r_20
tcp_outgoing_address x.x.0.19 r_19
tcp_outgoing_address x.x.0.18 r_18
tcp_outgoing_address x.x.0.17 r_17
tcp_outgoing_address x.x.0.16 r_16
tcp_outgoing_address x.x.0.15 r_15
tcp_outgoing_address x.x.0.14 r_14
tcp_outgoing_address x.x.0.13 r_13
tcp_outgoing_address x.x.0.12 r_12
tcp_outgoing_address x.x.0.11 r_11
tcp_outgoing_address x.x.0.10 r_10
tcp_outgoing_address x.x.0.9 r_9
tcp_outgoing_address x.x.0.8 r_8
tcp_outgoing_address x.x.0.7 r_7
tcp_outgoing_address x.x.0.6 r_6
tcp_outgoing_address x.x.0.5 r_5
tcp_outgoing_address x.x.0.4 r_4
tcp_outgoing_address x.x.0.3 r_3
tcp_outgoing_address x.x.0.2 r_2
tcp_outgoing_address x.x.0.1 r_1