After several hours of work in search engines and alterations, the problem was how the schema files were referenced: although WSDL and XSD were locally saved, there are still links to w3.org that looked like this:
<!DOCTYPE schema PUBLIC "-//W3C//DTD XMLSchema 200102//EN" "http://www.w3.org/2001/XMLSchema.dtd" [... <import namespace="http://www.w3.org/2000/09/xmldsig#" schemaLocation="http://www.w3.org/TR/2002/REC-xmldsig-core-20020212/xmldsig-core-schema.xsd" />
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w3.org was very slow, so my client was slow to initialize.
I changed the link to local:
<import namespace="http://www.w3.org/2000/09/xmldsig#" schemaLocation="xmldsig-core-schema.xsd" />
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