VPN speed reduces drastically over cellular personal hotspot

RouterOS Beginner Basics
1

Hi,
Model: RB750Gr3
Upgrade Firmware: 7.11.2
At home I use my Iphone (cellular VPN connection) to connect to the office network. The internet speed (in the Iphone) of this connection is very good (about 70Mbps).
Then I stop using VPN in the Iphone and try to use the same VPN (L2TP) connection in my Laptop (internet I share from this Iphone using Personal Hotspot).
Before connecting to VPN, the internet speed in the laptop is very good (about 70 Mbps), but when I connect to VPN the internet speed reduces drastically (sometimes less than 1Mbps).
Can you help to solve this speed issue?

Thanks!

2

Probably nothing to do with your Mikrotik router ?

What type of VPN ? I assume you launch it on your laptop ?
Can you make a small drawing indicating all mentioned pieces and how they interact with each other ?
Because since you mention hotspot on your phone, what has Hex still to do then ??

3
  1. From anywhere in the city my iPhone acts as an access point (basically becomes the Wi-Fi router). The internet speed (in the iPhone) is good.
    1.png
  2. Then my PC (laptop) connects to this Wi-Fi iPhone “hotspot/access point” and browses the internet. The internet speed (in the PC) is good.
    2.png
  3. In my PC I have a VPN ( L2TP) connection to connect to Mikrotik router to connect to my office network. I connect to this VPN connection. The internet download speed (in the PC) reduces drastically.
    5.png
  4. In my PC I have a VPN ( PPTP) connection to connect to Mikrotik router to connect to my office network. I can’t connect to this VPN connection at all using this topology. I got an error.
    4.png
  5. In the same iPhone I have a VPN (L2TP) connection to the same router. I connect to this connection and there is no problem with the speed.
    3.png
    I tried to use different iPhone – the result was the same.

Internet speed in my office is very good.

Other scenario:

  1. At home I use the regular wifi in my PC (laptop). The internet speed (in the PC) is quite good.
    8.png
  2. In my PC I have a VPN ( L2TP) connection to connect to Mikrotik router to connect to my office network. I connect to this VPN connection. The internet download speed (in the PC) reduces drastically.
    7.png
  3. In my PC I have a VPN ( PPTP) connection to connect to Mikrotik router to connect to my office network I connect to this VPN connection. The internet speed (in the PC) remains practically the same.
    6.png
    There is a problem with (L2TP) connection download speed and VPN (PPTP) connection when I am using my iPhone.

Can you help to solve them?