This might be limited to .ods files - .xlsx files with the same content do not suffer the problem. A quick check of the soffice.bin shows that .ods import is spending all its time in XML_ParseBuffer() and its children. Sample stack trace:

(gdb) bt
#0  0x00007f8c7044868a in ?? ()
   from /usr/lib/libreoffice/program/../program/libeditenglo.so
#1  0x00007f8c70449d33 in ?? ()
   from /usr/lib/libreoffice/program/../program/libeditenglo.so
#2  0x00007f8c70457c74 in ?? ()
   from /usr/lib/libreoffice/program/../program/libeditenglo.so
#3  0x00007f8c704583cc in ?? ()
   from /usr/lib/libreoffice/program/../program/libeditenglo.so
#4  0x00007f8c7045c2f4 in ?? ()
   from /usr/lib/libreoffice/program/../program/libeditenglo.so
#5  0x00007f8c7041877c in EditEngine::SetUpdateMode(unsigned char) ()
   from /usr/lib/libreoffice/program/../program/libeditenglo.so
#6  0x00007f8c71481187 in ?? ()
   from /usr/lib/libreoffice/program/../program/libsclo.so
#7  0x00007f8c70503ba9 in SvxUnoTextCursor::gotoRange(com::sun::star::uno::Reference<com::sun::star::text::XTextRange> const&, unsigned char) ()
   from /usr/lib/libreoffice/program/../program/libeditenglo.so
#8  0x00007f8c72734f12 in ?? ()
   from /usr/lib/libreoffice/program/../program/libxolo.so
#9  0x00007f8c727366d9 in ?? ()
   from /usr/lib/libreoffice/program/../program/libxolo.so
#10 0x00007f8c72533fe3 in SvXMLImport::endElement(rtl::OUString const&) ()
   from /usr/lib/libreoffice/program/../program/libxolo.so
---Type <return> to continue, or q <return> to quit---
#11 0x00007f8c721f236d in ?? ()
   from /usr/lib/libreoffice/program/../program/expwrap.uno.so
#12 0x00007f8c92714454 in ?? () from /lib/x86_64-linux-gnu/libexpat.so.1
#13 0x00007f8c9271554e in ?? () from /lib/x86_64-linux-gnu/libexpat.so.1
#14 0x00007f8c9271985d in XML_ParseBuffer ()
   from /lib/x86_64-linux-gnu/libexpat.so.1
#15 0x00007f8c721f0c16 in ?? ()
   from /usr/lib/libreoffice/program/../program/expwrap.uno.so
#16 0x00007f8c721f49ee in ?? ()
   from /usr/lib/libreoffice/program/../program/expwrap.uno.so
#17 0x00007f8c711d8c9d in ?? ()
   from /usr/lib/libreoffice/program/../program/libsclo.so
#18 0x00007f8c711dac87 in ?? ()
   from /usr/lib/libreoffice/program/../program/libsclo.so
#19 0x00007f8c7129f502 in ?? ()
   from /usr/lib/libreoffice/program/../program/libsclo.so
#20 0x00007f8c7129f8bd in ScDocShell::Load(SfxMedium&) ()
   from /usr/lib/libreoffice/program/../program/libsclo.so
#21 0x00007f8c9ac53d0d in SfxObjectShell::LoadOwnFormat(SfxMedium&) ()
   from /usr/lib/libreoffice/program/libsfxlo.so
#22 0x00007f8c9ac60c45 in SfxObjectShell::DoLoad(SfxMedium*) ()
   from /usr/lib/libreoffice/program/libsfxlo.so
#23 0x00007f8c9aca3d8d in SfxBaseModel::load(com::sun::star::uno::Sequence<com::---Type <return> to continue, or q <return> to quit---
sun::star::beans::PropertyValue> const&) ()
   from /usr/lib/libreoffice/program/libsfxlo.so
#24 0x00007f8c9acdd520 in ?? () from /usr/lib/libreoffice/program/libsfxlo.so
#25 0x00007f8c7afc76b3 in ?? ()
   from /usr/lib/libreoffice/program/../program/libfwklo.so
#26 0x00007f8c7afc8af8 in ?? ()
   from /usr/lib/libreoffice/program/../program/libfwklo.so
#27 0x00007f8c7af4134e in ?? ()
   from /usr/lib/libreoffice/program/../program/libfwklo.so
#28 0x00007f8c7af41838 in ?? ()
   from /usr/lib/libreoffice/program/../program/libfwklo.so
#29 0x00007f8c9bc4ff94 in comphelper::SynchronousDispatch::dispatch(com::sun::star::uno::Reference<com::sun::star::uno::XInterface> const&, rtl::OUString const&, rtl::OUString const&, int, com::sun::star::uno::Sequence<com::sun::star::beans::PropertyValue> const&) ()
   from /usr/lib/libreoffice/program/libcomphelpgcc3.so
#30 0x00007f8c9cf0dd47 in ?? ()
   from /usr/lib/libreoffice/program/libsofficeapp.so
#31 0x00007f8c9cf1b661 in ?? ()
   from /usr/lib/libreoffice/program/libsofficeapp.so
#32 0x00007f8c9cef0229 in ?? ()
   from /usr/lib/libreoffice/program/libsofficeapp.so
#33 0x00007f8c9cef15dc in ?? ()
---Type <return> to continue, or q <return> to quit---
   from /usr/lib/libreoffice/program/libsofficeapp.so
#34 0x00007f8c98e6dd14 in ?? () from /usr/lib/libreoffice/program/libvcllo.so
#35 0x00007f8c98e768bc in SalGenericDisplay::DispatchInternalEvent() ()
   from /usr/lib/libreoffice/program/libvcllo.so
#36 0x00007f8c8faa975f in ?? ()
   from /usr/lib/libreoffice/program/libvclplug_gtklo.so
#37 0x00007f8c8faa97d9 in ?? ()
   from /usr/lib/libreoffice/program/libvclplug_gtklo.so
#38 0x00007f8c8da32c65 in g_main_context_dispatch ()
   from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#39 0x00007f8c8da32f98 in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#40 0x00007f8c8da33054 in g_main_context_iteration ()
   from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#41 0x00007f8c8faa94f1 in ?? ()
   from /usr/lib/libreoffice/program/libvclplug_gtklo.so
#42 0x00007f8c98be6854 in Application::Yield(bool) ()
   from /usr/lib/libreoffice/program/libvcllo.so
#43 0x00007f8c98be68f7 in Application::Execute() ()
   from /usr/lib/libreoffice/program/libvcllo.so
#44 0x00007f8c9ceee1a4 in ?? ()
   from /usr/lib/libreoffice/program/libsofficeapp.so
#45 0x00007f8c98beee49 in ?? () from /usr/lib/libreoffice/program/libvcllo.so
#46 0x00007f8c98beeed5 in SVMain() ()
---Type <return> to continue, or q <return> to quit---
   from /usr/lib/libreoffice/program/libvcllo.so
#47 0x00007f8c9cf1ca16 in soffice_main ()
   from /usr/lib/libreoffice/program/libsofficeapp.so
#48 0x00000000004006bb in ?? ()
#49 0x00007f8c9c32376d in __libc_start_main ()
   from /lib/x86_64-linux-gnu/libc.so.6
#50 0x00000000004006f1 in ?? ()
#51 0x00007fff5a6b01c8 in ?? ()
#52 0x000000000000001c in ?? ()
#53 0x0000000000000004 in ?? ()
#54 0x00007fff5a6b1864 in ?? ()
#55 0x00007fff5a6b188d in ?? ()
#56 0x00007fff5a6b1894 in ?? ()
#57 0x00007fff5a6b18cc in ?? ()
#58 0x0000000000000000 in ?? ()
(gdb)