How can I reduce the boot time caused by dkms.service and numerous serial device initializations in systemd?

I'm experiencing long boot times on my Linux system, and using systemd-analyze blame, I've identified that dkms.service and a number of serial device initializations are the main culprits:

➜  ~ systemd-analyze critical-chain
The time when unit became active or started is printed after the "@" character.
The time the unit took to start is printed after the "+" character.

graphical.target @1min 55.130s
└─multi-user.target @1min 55.130s
  └─plymouth-quit-wait.service @1min 51.711s +3.415s
    └─systemd-user-sessions.service @1min 51.661s +14ms
      └─remote-fs.target @1min 51.645s
        └─remote-fs-pre.target @1min 47.858s
          └─nfs-client.target @1min 47.858s
            └─gssproxy.service @1min 47.825s +31ms
              └─network.target @1min 47.812s
                └─wpa_supplicant.service @1min 47.797s +13ms
                  └─basic.target @2.246s
                    └─dbus-broker.service @2.220s +23ms
                      └─dbus.socket @2.199s
                        └─sysinit.target @2.194s
                          └─systemd-resolved.service @2.081s +112ms
                            └─systemd-tmpfiles-setup.service @1.975s +90ms
                              └─import-state.service @1.908s +27ms
                                └─local-fs.target @1.892s
                                  └─boot-efi.mount @1.867s +24ms
                                    └─boot.mount @1.848s +10ms
                                      └─systemd-fsck@dev-disk-by\x2duuid-22b66811\x2dda34\x2d4a10\x2d880a\x2dff18d03fb3a4.service @1.175s +32ms
                                        └─dev-disk-by\x2duuid-22b66811\x2dda34\x2d4a10\x2d880a\x2dff18d03fb3a4.device @584542y 2w 2d 20h 1min 47.565s +3.159s

The dkms.service alone is taking over a minute and a half, and many serial devices (ttyS*) are each taking around 3.4 seconds to initialize.