我的 Ubuntu 安装需要花费大量时间来启动。我有一台 MacBook Air 2014，其中的 Ubuntu 16.04 安装在 USB 上。根据dmesg延迟情况如下：
[ 3.166478] sd 1:0:0:0: Attached scsi generic sg1 type 0
[ 3.167125] sd 1:0:0:0: [sdb] 60063744 512-byte logical blocks: (30.8 GB/28.6 GiB)
[ 3.168958] sd 1:0:0:0: [sdb] Write Protect is off
[ 3.168962] sd 1:0:0:0: [sdb] Mode Sense: 43 00 00 00
[ 3.169826] sd 1:0:0:0: [sdb] Write cache: disabled, read cache: enabled, doesn't support DPO or FUA
[ 3.202890] sdb: sdb1 sdb2 < sdb5 >
[ 3.205129] sd 1:0:0:0: [sdb] Attached SCSI removable disk
[ 3.548151] clocksource: Switched to clocksource tsc
[ 3.581976] scsi 2:0:0:0: Direct-Access APPLE SD Card Reader 3.00 PQ: 0 ANSI: 6
[ 3.582455] sd 2:0:0:0: Attached scsi generic sg2 type 0
[ 3.583683] sd 2:0:0:0: [sdc] Attached SCSI removable disk
[ 20.022675] EXT4-fs (dm-0): mounted filesystem with ordered data mode. Opts: (null)
[ 20.403096] systemd[1]: systemd 229 running in system mode. (+PAM +AUDIT +SELINUX +IMA +APPARMOR +SMACK +SYSVINIT +UTMP +LIBCRYPTSETUP +GCRYPT +GNUTLS +ACL +XZ -LZ4 +SECCOMP +BLKID +ELFUTILS +KMOD -IDN)
[ 20.420092] systemd[1]: Detected architecture x86-64.
任何想法如何解决这一问题？