Ubuntu Mate 18.04 on MacbookPro Retina: slow start on cold boot

Hello! I recently installed Ubuntu Mate 18.04 on a 2015 Macbook Pro 13 Retina with a 500GB SSD and have encountered a strange problem.

On a “cold boot” (complete power down and then pressing the power button to start), it takes about 2 min 30sec from power on to get to the login screen. There’s always a gray screen that flashes once during that time. I think this is unusually slow but I don't know how to make it faster.

But if I restart the Macbook from within Ubuntu Mate, it restarts and loads the login screen in about 30 sec or so, which is what I think is normal.

I’ve done web searches but I can’t find any solutions. This isn't a suspension problem (I saw there's another forum thread about MBPro and suspension, but I don't face that problem).

Please advise? Thanks in advance!

Here is the output of my inxi -Fxz

System: Host: wgui-MacBookPro Kernel: 4.18.0-16-generic x86_64
bits: 64 gcc: 7.3.0
Desktop: MATE 1.20.1 (Gtk 3.22.30-1ubuntu2)
Distro: Ubuntu 18.04.2 LTS
Machine: Device: laptop System: Apple product: MacBookPro12 1 v: 1.0 serial: N/A
Mobo: Apple model: Mac-E43C1C25D4880AD6 v: MacBookPro12 1 serial: N/A
UEFI: Apple v: MBP121.88Z.0167.B14.1509281135 date: 09/28/2015
Battery BAT0: charge: 72.2 Wh 100.0% condition: 72.2/74.8 Wh (96%)
model: SMP bq20z451 status: Full
CPU: Dual core Intel Core i5-5287U (-MT-MCP-)
arch: Broadwell rev.4 cache: 3072 KB
flags: (lm nx sse sse2 sse3 sse4_1 sse4_2 ssse3 vmx) bmips: 11599
clock speeds: max: 3300 MHz 1: 2347 MHz 2: 1780 MHz 3: 2733 MHz
4: 2461 MHz
Graphics: Card: Intel Iris Graphics 6100 bus-ID: 00:02.0
Display Server: x11 (X.Org 1.20.1 )
drivers: modesetting (unloaded: fbdev,vesa)
Resolution: [email protected]
OpenGL: renderer: Mesa DRI Intel Iris 6100 (Broadwell GT3)
version: 4.5 Mesa 18.2.8 Direct Render: Yes
Audio: Card-1 Intel Broadwell-U Audio Controller
driver: snd_hda_intel bus-ID: 00:03.0
Card-2 Intel Wildcat Point-LP High Def. Audio Controller
driver: snd_hda_intel bus-ID: 00:1b.0
Sound: Advanced Linux Sound Architecture v: k4.18.0-16-generic
Network: Card: Broadcom and subsidiaries BCM43602 802.11ac Wireless LAN SoC
driver: brcmfmac bus-ID: 03:00.0
IF: wlp3s0 state: up mac:
Drives: HDD Total Size: 500.3GB (2.4% used)
ID-1: /dev/sda model: APPLE_SSD_SM0512 size: 500.3GB
Partition: ID-1: / size: 458G used: 12G (3%) fs: ext4 dev: /dev/sda2
RAID: No RAID devices: /proc/mdstat, md_mod kernel module present
Sensors: System Temperatures: cpu: 33.9C mobo: N/A
Fan Speeds (in rpm): cpu: N/A
Info: Processes: 197 Uptime: 6 min Memory: 730.9/7879.7MB
Init: systemd runlevel: 5 Gcc sys: N/A
Client: Shell (bash 4.4.191) inxi: 2.3.56