Update
Problem fixed with restarting Linode. Probably it was some updates in the system or server that needs a good flush. I noticed it uses more RAM for working with daily tasks. Planning to use NextCloud talk soon, so I think this will be a CPU intensive feature. Will soon upgrade my VPS.
If you have any affordable and good VPS, do give a shout 
@timconsidine Hi! Thank you so much for the reply & amazing tips! Sorry for getting back so late, I only saw a notification for replies to my email today.
Yes you are right, I restarted the Linode and it worked much much better now.
Currently there are only 3 of us in the team, but we also want to share some files and work with clients with Nextcloud. 4GB will get choked pretty soon.
And wow SSDNODES really have good price.
Most of my websites or web apps are in Linode. As you said, it's expensive. I was thinking about Contabo VPS M, but I heard mixed reviews about them. I'll give them KVM / X-LARGE a try.
For storage: Wasabi - yes I've tried the trial too, they seem quite good and price is reasonable, will try to integrate them sometime next week.
@humptydumpty You are right on this point, but that was before the slowness started, I had to remove an app. Currently I only have the featured ones like deck cards, notes, and kanban add-on. I've recently tried out Collabora but I think it's resource hungry too, so I removed for now, and will add it back once I've upgraded my VPS. I did not allocate any RAM for NextCloud in Cloudron, I thought it would pull more if it needed more. Sometimes I don't run it, and use other apps.
@scooke Yes, I've checked this post earlier, I think my problem had more to do with resources that NextCloud needs vs what I have. But restarting helps.