I joined a lot of pocketmine hosted servers with many players and most of them seems to lag quite a lot but when i joined AvengeTech PE it was running fine at 70/70 players
It seems AvengeTech uses an OVH dedicated server (the same I use), so this is probably the hardware they use: Intel i7-6700K 4c/8t - 4GHz /4.2GHz 32GB DDR4 2133 MHz SoftRaid 2x480GB SSD 250 Mbps bandwidth There is no way you'll be able to get similar performance on a VPS, even less so on turnkey host such as NetherBox. What you may be able to do is upgrade your current hosting with an SSD for example, or a faster CPU. What are your current specs?
It's been a while since I had 100 players in a server... but at that time the busiest was lag-free up to about 80, yes.
Write good code and pay attention to quality. Hardware is important too, but you can make any server's performance drop off a cliff by piling bad code on top of it. All the CPU power in the world won't make accessing the internet any faster, for example.
It also depends on your server's gamemode. A good VPS might do for a 70-player minigame server provided your code quality is above average. For factions and related, you'll need better hardware.
Keep in mind it might be your ping to cosmicPE. Someone who lives closer to the location it's hosted would have a lower ping, and lag less. Someone farther away would have a higher ping and would lag more. It also depends on the speed of your internet connection.
Fair point, it's currently hosted near to where most of the players join from. I've seen some servers have AU, EU and US servers to combat the ping problem. Spoiler Best wrt server load