How do I solve the problem the eigenvalue is very slow to be solved analytically？

Hello, I'm very upset this code doen't run a result,it just running all the time.I think the reason is the analytic derivation process which need a analytic solution of matrix W. I hope someone would give me some help or point the problems,I will be very grateful.

tic;
clc;
clear;
syms B
W11=fa(B,1,1,1,1);W12=fa(B,1,1,1,0);W13=fa(B,1,1,0,1);W14=fa(B,1,1,0,0);
W21=fa(B,1,0,1,1);W22=fa(B,1,0,1,0);W23=fa(B,1,0,0,1);W24=fa(B,1,0,0,0);
W31=fa(B,0,1,1,1);W32=fa(B,0,1,1,0);W33=fa(B,0,1,0,1);W34=fa(B,0,1,0,0);
W41=fa(B,0,0,1,1);W42=fa(B,0,0,1,0);W43=fa(B,0,0,0,1);W44=fa(B,0,0,0,0);
W=simplify([W11,W12,W13,W14;W21,W22,W23,W24;W31,W32,W33,W34;W41,W42,W43,W44]);
E1=eig(W);
E2=sort(E1);
E3=E2(4);
T=1;
be=1/T;
f1=-1/be*log(E3);
ma=-diff(f1);
m1=subs(ma,{B},2)
toc;

function s1=fa(B,m1i,m1j,m2i,m2j)
JH=2;J=1;T=1;de=0.2;
sx=1/2*[0 1;1 0];
sy=1/2*[0 -1i;1i 0];
sz=1/2*[1 0;0 -1];
u=eye(2);
be=1/T;
H11=kron(sx,kron(sx,kron(u,u)))+kron(sy,kron(sy,kron(u,u)))...
+de*kron(sz,kron(sz,kron(u,u)));
H12=kron(u,kron(sx,kron(sx,u)))+kron(u,kron(sy,kron(sy,u)))...
+de*kron(u,kron(sz,kron(sz,u)));
H2=kron(sz,kron(u,kron(u,u)))+kron(u,kron(sz,kron(u,u)));
H3=kron(u,kron(u,kron(u,u)));

H=-JH*(H11+H12)+J*H2*(m1i+m1j)+1/2*J*H3*(m1i+m2i+m1j+m2j)...
    -B*1/2*H3*(m1i+m2i+m1j+m2j);
va=eig(H);
s1=sum(exp(-be*va));
end

[Christine Tobler]

Solving eigenvalue problems (even just of size 4) analytically is quite expensive. If you just set B = 2 from the beginning, I think this should be much faster. What is the reason to first need to compute the analytical solution for general B, and then substitute into it?