Suggestion about how to proceed with installation

Laptop specs :
System Model : Dell Inspiron 7577
Processor : i5-7300 HQ
SSD : 256 GB NVME SSD
HDD : 1TB
I tried allocating 100 GB space on HDD and installed Zorin OS Pro there. I noticed boot time is slow close to 2 minutes.
Tried next option . root on ssd and home on HDD but I got some installation error.
My question is if i just install on hdd, does it impact app opening speed too . I am fine to do compromise with boot speed but not later

Technically yes. Launching an app when installed on SSD is faster than when installed in HDD. However... it is such a slight difference, you would not even notice it.
The real difference is in Boot speed. SSD will boot much faster than HDD.
SSD shows faster (and definitely noticeable) speeds on very large files and boot but not on app launching or basic functions.

Okay I will stick with this and see how it goes

AGREED!

The standard for today, is to install your operating system, on a SSD drive. This not only aids in greater speeds, weather loading into the OS, APPS, games, but it produces less heat generation, and less power is consumed per watt, that the system is on.

SSD's have been the wave of the future for so long, that now they got these new micro SSD's now that are starting to become a thing. Anyways, you get my vote for switching to SSD.

Which SSD should you get I hear you ask. Well, I am so glad you asked me that! I recommend Samsung SSD drives. If your looking to save some money, the Samsung EVO drives will do perfectly fine.

If you don't do a bunch of gaming, nor a bunch of production work, you could get away with a 256GB to 512GB SSD drive. Those can be had for less then 100 dollars. If you plan to do gaming or production work, I recommend at least a 1TB drive minimum, those can be right now for around 150 dollars.