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ChainComplex() runs 24 times slower than homology()

I load a list of matrices bdrs representing a chain complex, their dimensions are {1, 21, 210, 1330, 5985, 20349, 54264, 116280, 203490, 293930, 352716, 352716, 293930, 203490, 116280, 54264, 20349, 5985, 1330, 210, 21, 1}, and the largest has density 3.91*10^-6. In total they take up 50MB of disk space. This finishes in 63sec.

When I run chcx=ChainComplex(bdrs,base_ring=GF(2)), it takes 7hrs20min, but chcx.homology() finishes in only 18min. Why does it take so long to just store a few matrices? At first I thought that ChainComplex() also does some simplifications/reductions, but [chcx.free_module_rank(i) for i in range(0,21)] shows the original dimensions of matrices :/.

Is there a faster way to compute the homology of a chain complex (over $\mathbb{Z}$ or $\mathbb{Z}_p$)?