In my experience, Tor through VPN services isn't substantially slower than Tor alone. I only know that from experiments using VPS, however, because I've never used Tor (or I2P or Freenet, for that matter) directly.
VPNs through Tor also aren't substantially slower than Tor alone. And indeed, one can use MPTCP to aggregate multiple VPN-via-Tor connections. But only between suitably configured devices, of course.
regardless of "substantially slower" it is indeed slower, I guess it depends on your VPN link, but at the very least your MTU has to be considerably smaller meaning more round trips for "large" objects (anything over 1KiB essentially)
VPNs through Tor also aren't substantially slower than Tor alone. And indeed, one can use MPTCP to aggregate multiple VPN-via-Tor connections. But only between suitably configured devices, of course.